When I was four and Cass was six, she whacked me across the face with a plastic shovel at our neighborhood park. We were in the sand box, and it was winter: In the pictures, we're in matching coats and hats and mittens. My mother loved to dress us alike, like twins, since we were only two years apart. We did look alike, with the same round face and dark eyes and the same brown hair. But we weren't the same, even then.

The story goes like this: Cass had the shovel and I wanted it. My mother was sitting watching us on a bench with Boo, who had her camera and was snapping pictures. This was at Commons Park, the small grassy area in the center of our neighborhood, Lakeview. Besides the sandboxes it also had a swing set, one of those circular things you push real fast and then jump on—a kind of manual merry-go-round— and enough grass to play baseball or kickball. Cass and I spent most of the afternoons of our childhood at Commons Park, but the shovel incident is what we both always remembered.

Not that we ourselves recalled it that well. We had just heard the story recounted so many times over the years that it was easy to take the details and fold them into our own sparse memories, embellishing here or there to fill in the blanks.

It is said that I reached for the shovel and Cass wouldn't give it to me, so I grabbed her hand and tried to yank it away. A struggle ensued, which must have looked harmless until Cass somehow scraped one hard plastic edge across my temple and it began to bleed.

This moment, the moment, we have documented in one of Boo's photos. There is one picture of Cass and me playing happily, another of the struggle over the shovel (I'm wailing, my mouth a perfect O, while Cass looks stubborn and determined, always a fighter), and finally, a shot of her arm extended, the shovel against my face, and a blur in the left corner, which I know is my mother, jumping to her feet and running to the sandbox to pull us apart.

Apparently, there was a lot of blood. My mother ran through the winding sidewalks of Lakeview with me in her arms, shrieking then took me to the hospital where I received five tiny stitches. Cass got to stay at Boo and Stewart's, eat ice cream, and watch TV until we got home.

The shovel was destroyed. My mother, already a nervous case, wouldn't let us leave the house or play with anything not plush or stuffed for about six months. And I grew up with a scar over my eye, small enough that hardly anyone even noticed it, except for me. And Cass.

As we grew older, I'd sometimes look up to find her peering very closely at my face, finding the scar with her eyes before reaching up with one hand to trace it with her finger. She always said it made her feel horrible to look at it, even though we both knew it wasn't really her fault. It was just one more thing we had in common, like our faces, our gestures, and our initials.

When Cass was born my mother still wasn't sure what to name her. My mother had suffered terrible morning sickness, and Boo, who had moved in next door during the fourth month or so, spent a lot of time making herbal tea and rubbing my mother's feet, trying to make her force down the occasional saltine cracker. Boo was the one who suggested Cassandra.

"In Greek mythology she was a seer, a prophet," she told my mother, whose tendencies leaned more toward Alice or Mary. "Of course she came to a horrible end, but in Greek mythology, who doesn't? Besides, what more could you want for your daughter than to be able to see her own future?"

So Cassandra it was. By the time I came along, my mom and Boo were best friends. Boo's real name was Katherine, but she hated it, so I was named Caitlin, the Irish version. Cass's name was always cooler, but to be named for Boo was something special, so I never complained. Her name was just one thing I envied about Cass. Even with all our similarities, it was the things we didn't have in common that I was always most aware of.

My sister wasn't a seer or a prophet, at least not at eighteen. What she was, was student body president two years running, star right wing of the girls' soccer team (State Champs her junior and senior year), and Homecoming Queen. She volunteered chopping vegetables at the homeless shelter for soup night every Thursday, had been skydiving twice, and was famous in our high school for staging a sit-in to protest the firing of a popular English teacher for assigning "questionable reading material"—Tony Morrison's Beloved. She made the local news for that one, speaking clearly and angrily to a local reporter, her eyes blazing, with half the school framed in the shot cheering behind her. My father, in his recliner, just sat there and grinned.

There were only two times I can remember ever seeing Cass really depressed. One was after the soccer State Championship sophomore year, when she missed the goal that could have won it all. She locked herself in her room for a full day. She never talked about it again, instead just focusing on the next season, when she rectified the loss by scoring the only two goals of the championship game. The second time was at the end of her junior year, when her first real boyfriend, Jason Packer, dumped her so he could "see other people" and "enjoy his freedom" in his last summer before college. Cass cried for a week straight, sitting on her bed in her bathrobe and staring out the window, refusing to go anywhere.

She drew back from everyone a bit, spending a lot of time next door with Boo where they drank tea, discussed Zen Buddhism, and read dream books together. This was when Cass became so spiritual, scanning the world around her for signs and symbols, sure that there had to be a message for her somewhere.

She got into three out of the four schools she applied to, and ended up choosing Yale. My parents were ecstatic and threw a party to celebrate. We all applauded and cheered as she bent over to slice a big cake that read WATCH OUT YALE: HERE COMES CASS! which my mother had ordered special from a bakery in town.

But Cass wasn't herself. She smiled and accepted all the pats on the back, rolling her eyes now and then at my parents' pride and excitement. But it seemed to me that she was just going through the motions. I wondered if she was looking for a sign, something she couldn't find with us or even at Yale.

She stayed in this funk all the way through graduation. In mid-June she went to stay with her friend Mindy's family at the beach and got a job renting out beach chairs by the boardwalk every day. Three mornings into it she met Adam. He was down at the beach on vacation with some friends from the show, and rented a chair from her. He stayed all day, then asked her out.

I could tell when she called the next morning, her voice so happy and laughing over the line, that our Cass was back. But not, we soon learned, for long.

I don't think any of us knew how much we'd needed Cass until she was gone. All we had was her room, her stories, and the quiet that settled in as we tried in vain to spread ourselves out and fill the space she'd left behind.

Everyone forgot my birthday as our kitchen became mission control, full of ringing phones, loud voices, and panic. My mother refused to leave the phone, positive Cass would call any minute and say it was all a joke, of course she was still going to Yale. Meanwhile my mother's friend from the PTA and Junior League circled through the house making fresh pots of coffee every five minutes, wiping the counters down, and clucking their tongues in packs by the back door. My father shut himself in his office to call everyone who'd ever known Cass, hanging up each time to cross another name off the long list in front of him. She was eighteen, so technically she couldn't be listed as a runaway. She was more like a soldier gone AWOL, still owing some service and on the lam.

They'd already tried Adam's apartment in New York, but the number had been disconnected. Then they called the Lamont Whipper Show, where they kept getting an answering machine encouraging them to leave their experience with this week's topic—My Twin Dresses Like a Slut and I Can't Stand It! —so that a staffer could get back to them.

"I can't believe she'd do this," my mother kept saying. "Yale. She's supposed to be at Yale." And all the heads around her would nod, or hand her more coffee, or cluck again.

I went into Cass's room and sat on her bed, looking around at how neatly she'd left everything. In a stack by the bureau was everything she and my mother had bought on endless Saturday trips to Wal-Mart for college: pillowcases, a fan, a little plastic basket to hold her shower stuff, and her new blue comforter, still in its plastic bag. I wondered how long she'd know she wouldn't use any of this stuff—when she'd hatched this plan to be with Adam. She'd fooled us all, every one.

She had come home from the beach tanned, gorgeous, and sloppy in love, and proceeded to spend about an hour each night on the phone long-distance with him, spending every bit of the money she'd made that summer.

"I love you," she'd whisper to him, and I'd blush; she didn't even care that I was there. She'd be lying across the bed, twirling and untwirling the cord around her wrist. "No, I love you more. I do. Adam, I do. Okay. I love you too. What? More than anything. Anything. I swear. Okay. I love you too." And when she finally did hang up she'd pull her legs up against her chest, grinning stupidly, and sigh.

"You are pathetic," I told her one night when it was particularly sickening, involving about twenty I love yous and four pumpkins.

"Oh, Caitlin," she said, sighing again, rolling over on the bed and sitting up to look at me. "Someday this will happen to you."

"God, I hope not," I said. "If I act like that, be sure to put me out of my misery."

"Oh, really," she said, raising one eyebrow. Then, before I could react, she lunged forward and grabbed me around the waist, pulling me down onto the bed with her. I tried to wriggle away but she was strong, laughing in my ear as we fought. "Give," she said in my ear; she had a lock hold on my waist. "Go on. Say it."

"Okay, okay," I said, laughing. "I give." I could feel her breathing against the back of my neck.

"Caitlin, Caitlin," she said in my ear, one arm still thrown over my shoulder, holding me there. She reached up with her finger and traced the scar over my eyebrow, and I closed my eye, breathing in. Cass always smelled like Ivory soap and fresh air. "You're such a pain in the ass," she whispered to me. "But I love you anyway."

"Likewise," I said.

That had been two weeks earlier. She had to have known even then she was leaving.

I walked to her mirror and looked at all the ribbons and pictures she had taped around it: spelling bees, honor roll, shots from the mall photo booth of her friends making faces and laughing, their arms looped around each other. There were a couple of us, too. One from a Christmas when we were kids, both of us in little red dresses and white tights, holding hands, and one from a summer at the lake where we're sitting at the end of a dock, legs dangling over, in our matching blue polka-dot bathing suits eating Popsicles.

On the other side of the wall, in my room, I had the same bed, the same bureau set, and the same mirror. But on my mirror, I had one picture of my best friend, Rina, my third-place ribbon from horseback riding, and my certificate from the B honor roll. Most people would have been happy with that. But for me, with Cass always blazing the trail ahead, there was nothing to do but pale in comparison.

Okay, so maybe I was jealous, now and then, but I could never have hated Cass. She came to all my competitions, cheering the loudest as I went for the bronze. She was the first one waiting for me when I came off the ice during my only skating competition, after falling on my ass four times in five minutes. She didn't even say anything, just took her mittens, gave them to me, and helped me back to the dressing rooms where I cried in private as she unlaced my skates, telling knock-knock jokes the whole time.

To be honest, a part of me had been looking forward to Cass going off to Yale at the end of the summer. I though her leaving might actually give me some growing room, a chance to finally strike out on my own. But this changed everything.

I'd always counted on Cass to lead me. She was out there somewhere, but she'd take her own route, and for once I couldn't follow. This time, she'd left me to find my own way.

The next morning when I woke up I realized I hadn't dreamed at all, not even one fleeting image. I took the book Cass gave me out from under my bed, where I'd hidden it, and opened it to the first page, There was a drawing of a full moon, sprinkled with stars, in the corner.

August 18, I wrote at the top of the page. Nothing last night. And you're still gone.

I couldn't think of anything else, so I got out of bed, threw on some clothes, and went down the hallway to the kitchen. The door to my parents' room was closed and my father was in his office, on the phone. He had to have talked to a hundred people in the last twenty-four hours.

"I understand that," he was saying, his voice level, but I could tell he was frustrated. "But eighteen or not, we want her home. She's not the kind of girl who does something like this."

The door to his office was half open, and I could see him standing by the window, running his palm over the small bald patch at the back of his head. My father, as the Dean of Students at the university, dealt with problems every day. He was the stand-in parent for thousands of undergraduates, and was quoted each time a fraternity got caught pulling pranks or a beer bash got out of hand. But this was different. This was about us.

I pulled the patio door open and slipped outside, where it was thickly hot and muggy, another August morning. But at least it was quiet.

Next door, I could see Boo and Stewart sitting at their kitchen table, eating breakfast. Boo raised her hand, waved, and then gestured for me to come over, smiling. I took one look back at my own house, where my mother's stress filled the rooms to the ceiling, leaving a stink and heaviness like smoke, and started across the one strip of green grass that separated their backyard from ours.

When I was little and got in trouble and sent to my room, I'd always sit on my bed and wish that Boo and Stewart were my parents. They'd never had kids of their own. My mother said it was because they acted so much like children themselves, but I liked to think it was so they could be there for me, if I ever needed to trade my own family.

The window in my room faced their back sunporch, an all-glass room where Boo kept most of her plants. She was mad for ferns. Stewart's studio—he taught art at the university—was just off that room, in what was supposed to be the living room. They kept their bed in the corner, and they didn't even have any real furniture to speak of; when you were invited over, you sat on big red velvet cushions decorated with sequins that Boo had picked up on a trip to India. This drove my conservative mother crazy, so Boo and Stewart almost always came to our house, where Mom could relax among the safety and comfort of her ottomans and end tables.

But that was what Cass and I love most about them: their house, their lives, even their names.

"Mr. Connell's my father, and he lives in California," Stewart always said. He was a mild and quiet man, quite brilliant, whose hair was always sticking straight up, like a mad scientist's, and flecked with various colors of paint.

For most of the nights of my life I could hear Stewart coming home late from his university studio, the brakes of his bike—they had an old VW bus, but it broke down constantly—squeaking all the way from the bridge down the street. He'd glide down the slope of their yard, under the clothesline, to the garage. Sometimes he forgot about the clothesline and almost killed himself, flying backward while the bike went on, unmanned, to crash against the garage door. You'd think they would have moved the clothesline after the second time or so. But they didn't.

"It's not the fault of the clothesline," Stewart explained to me one day, rubbing the red, burned spot on his neck. He'd broken his glasses again and had them taped together in the middle. "It's about me respecting it as an obstacle."

Now Boo slid their door open and came out to meet me on their patio. She was in a pair of old overalls, a faded red tank top underneath, and her feet were bare. Her long red hair was piled on top of her head, a few chopsticks stuck in here and there to hold it in place. Inside, Stewart was sitting at the table, eating a big peach and reading a book. He looked up and waved at me; he had juice all over his chin.

"So," Boo said, putting an arm around my shoulder. "How are things on the home front?"

"Awful," I said. "Mom won't stop crying."

She sighed, and we stood there for a few minutes, just looking across their yard. Boo had gone through a Japanese garden stage a few years back, which resulted in a footbridge and a fat, rusted iron Buddha sculpture.

"I just can't believe she didn't tell me anything," I said. "I feel like I should have known something was going on."

Boo sighed, reaching to tuck a piece of hair behind her ear. "I think she probably didn't want to put you in that position," she said, squatting down to pull a dandelion at the edge of the patio, lifting it to her face to breathe in the scent. "It was a big secret to keep."

"I guess." Someone was mowing their lawn a few yards down, the motor humming. "I just thought everything was perfect for her, like it always was. You know?"

Boo nodded, standing up and stretching her back. "Well, that's a lot of pressure. Being perfect. Right?"

I shrugged. "I wouldn't know."

"Me neither," she said with a smile. "But I think it was harder for Cass than we realized, maybe. It's so easy to get caught up in what people expect of you. Sometimes, you can just lose yourself."

She walked to the edge of the patio, bending down to pull another dandelion. I watched her, then said, "Boo?"

"Yes?"

"Did she tell you she was going?"

She stood up slowly. "No," she said, as the lawnmower droned on down the street. "She didn't. But Cass had a hard year, last year. Things weren't always as easy as she made them seem, Caitlin. It's important that you know that."

I watched her pull a few more flowers, adding them to the bunch in her hand, before she came over and squeezed my shoulder. "What a crappy birthday, huh?" she said.

I shrugged. "It doesn't matter. I wouldn't have done anything anyway."

"What about Rina?" she said.

"She's off with her new stepdad," I told her, and she shook her head. "Bermuda this time." My best friend Rina Swain's mom had just gotten remarried again: This was number four. She only married rich, and never for love, which led to Rina living in nicer and nicer houses, going to endless exotic places, and piling up huge therapy bills. Rina had what Boo called Issues, but the guys at school had another name for it.

"Well, come inside," Boo said, pulling the door open and stepping back to met me in first. "Let me make you breakfast and we'll not talk about any of this at all."

I sat down at the table next to Stewart, who had finished his peach and was now sketching on the back of the power bill envelope, while Boo filled a mason jar with water and arranged the dandelions in it. Stewart's canvases, both finished and unfinished, covered the walls and were stacked against any solid surface in the house. Stewart did portraits of strangers: All his work was based on the theory that art was about the unfamiliar.

Stewart might have been unconventional, but his art classes were insanely popular at the university. This was mostly because he didn't believe in grades or criticism, and was a strong proponent of coed massage as a way of getting in touch with your artistic spirit. My father had been quoted about Stewart's teaching practices more than once, and always used words like unique, free spirit, and artistic choice. Privately, he wished Stewart would wear a tie now and then and stop leading meditation workshops in the quad on big football weekends.

Stewart looked over and smiled at me. "How's it feel to be sixteen?"

"No big difference," I said. With all the confusion, my father had forgotten about taking me to get my driver's license, but everyone had been so crazy I hadn't wanted to ask.

"Oh now," he said, pushing the envelope away and putting down his pen. "That's the great thing about aging. It just gets better every year."

"Here you go," Boo said, plunking a plate down in front of me: scrambled tofu, Fakin' Bacon, and some pomegranates.

"I remember when I was sixteen," Steward said, sitting back in his chair. His feet were bare, too, and sprinkled with green paint. "I hitched a ride to San Francisco and had a burrito for the first time. It was incredible."

"Really," I said, picking up the envelope he'd been doodling on. It was just half a face, sketchily drawn. I turned it over and was startled to see something in Cass's writing: her name, doodled in blue, signed with a flourish, as if she'd been sitting in this same chair some other morning, eating scrambled tofu, just like me.

"Just being free, out on the road, the world wide open…" He leaned closer to me, but I was still looking at Cass's name, suddenly so sad I felt like I couldn't breathe. It seemed impossible that Cass had been planning to change her life completely while none of us even noticed; even when she doodled on that envelope, it could have been on her mind.

"…anything possible," Stewart was saying. "Anything at all."

I blinked, and swallowed over the lump in my throat. I wanted to keep that envelope and hold it close to me, like it was suddenly all I had left of her, some sort of living part pulsing in my hand.

"Caitlin?" Boo said, coming over and bending down beside me. "What is it?" She leaned down and saw the envelope, catching her breath. "Oh honey" she said, and even before she wrapped her arms around me I was already leaning in, tucking my head against her shoulder as she held me, as I knew she'd held Cass, in this same chair, at this same table, in this same light, on other mornings, not like this.

When I walked up to our sliding glass door, the phone was ringing. No one seemed to be around, so I picked it up.

"Hello?"

There was silence, with just a bit of buzzing.

"Hello?"

My father appeared in the doorway, out of breath: He'd been outside, in the garage. "Who is it?"

I shook my head. "I don't—"

He was immediately beside me, pulling the receiver out of my hand. "Cassandra? Is that you?"

"Jack?" my mother said from their bedroom. I could hear her moving, coming closer, and then she appeared in the hallway, clutching a tissue, one hand over her mouth. "I dozed off. Is it—"

"Cassandra, listen to me. You have to come home. We're not mad at you, but you have to come home." His voice was shaking.

"Let me talk to her," my mother said, coming closer, but he shook his head, holding out one hand to keep her there.

"Tell her we love her!" my mother said, and I couldn't stand the way her voice sounded, unsure and wavering. I slipped around them both and into my room, slowly picking up my own phone. On the line, no one was speaking.

"Cassandra," my father said finally. "Talk to me."

Silence. I pictured her standing in a phone booth by a highway, cars whizzing by. A place I'd never seen, a world I didn't know. Then, suddenly, I heard her voice.

"Daddy," she began, and I heard my father take in a breath, quickly, as if he'd been punched in the stomach. "I'm okay. I'm happy. But I'm not coming home.""Where are you?" he demanded.

"Let me talk to her!" my mother shrieked in the background. She could have gone into my father's office and picked up the extension there, but I knew she wasn't thinking of that, couldn't even move from that spot in the hallway where she was standing. "Cassandra!"

"Don't worry about me," Cass said. "I'm—"

"No," my father said. "You must come home."

"This is what I want," she said. "You have to respect that."

"You're only eighteen," my father told her. "This is ridiculous, you can't possibly know—"

"Daddy," she said, and I realized suddenly I was crying, again, the receiver wet against my face. "I'm sorry. I love you. Please tell Mom not to worry."

"No," my father said, firm. "We are not—"

"Caitlin?" she said suddenly. "I know you're there. I can hear you."

"What is she saying?" my mother kept asking, now close to the receiver. "Where is she?"

"Margaret, just hold on," my father told her.

"Yes," I whispered back to Cass. "I'm here."

"Don't cry, okay?" she said. The line crackled, and I thought of her tackling me that night, her breath against my neck, laughing in my ear. "I love you. I'm sorry about your birthday."

"It's nothing," I said.

There was a voice outside her end, a yell, and another buzz on the line. "Is that him?" my father demanded. "Is he there?"

"I have to go," she said. "Please don't worry, okay?"

"Dammit, Cassandra," my father said. "Don't you hang up this phone!"

"Good-bye," she said softly, as my father's voice dropped away. "Good-bye."

"Cassandra!" my mother wailed into the phone, all the anger and fear of the last twenty-four hours bursting across the line. "Please—"

Click. And she was gone.