No one starts a war—or rather, no one in his sense ought to do so—without first being clear in his mind what he intends to achieve by that war and how he intends to conduct it.

—CARL VON CLAUSEWITZ, Vom Kriege

In my first memory, I am three years old and I am trying to kill my sister. Sometimes the recollection is so clear I can remember the itch of the pillowcase under my hand, the sharp point of her nose pressing into my palm. She didn't stand a chance against me, of course, but it still didn't work. My father walked by, tucking in the house for the night, and saved her. He led me back to my own bed. "That," he told me, "never happened."

As we got older, I didn't seem to exist, except in relation to her. I would watch her sleep across the room from me, one long shadow linking our beds, and I would count the ways. Poison, sprinkled on her cereal. A wicked undertow off the beach. Lightning striking.

In the end, though, I did not kill my sister. She did it all on her own.

Or at least this is what I tell myself.

WHEN I WAS LITTLE, the great mystery to me wasn't how babies were made, but why. The mechanics I understood—my older brother Jesse had filled me in—although at the time I was sure he'd heard half of it wrong. Other kids my age were busy looking up the words penis and vagina in the classroom dictionary when the teacher had her back turned, but I paid attention to different details. Like why some mothers only had one child, while other families seemed to multiply before your eyes. Or how the new girl in school, Sedona, told anyone who'd listen that she was named for the place where her parents were vacationing when they made her ("Good thing they weren't staying in Jersey City," my father used to say).

Now that I am thirteen, these distinctions are only more complicated: the eighth-grader who dropped out of school because she got into trouble; a neighbor who got herself pregnant in the hopes it would keep her husband from filing for divorce. I'm telling you, if aliens landed on earth today and took a good hard look at why babies get born, they'd conclude that most people have children by accident, or because they drink too much on a certain night, or because birth control isn't one hundred percent, or for a thousand other reasons that really aren't very flattering.

On the other hand, I was born for a very specific purpose. I wasn't the result of a cheap bottle of wine or a full moon or the heat of the moment. I was born because a scientist managed to hook up my mother's eggs and my father's sperm to create a specific combination of precious genetic material. In fact, when Jesse told me how babies get made and I, the great disbeliever, decided to ask my parents the truth, I got more than I bargained for. They sat me down and told me all the usual stuff, of course—but they also explained that they chose little embryonic me, specifically, because I could save my sister, Kate. "We loved you even more," my mother made sure to say, "because we knew what exactly we were getting."

It made me wonder, though, what would have happened if Kate had been healthy. Chances are, I'd still be floating up in Heaven or wherever, waiting to be attached to a body to spend some time on Earth. Certainly I would not be part of this family. See, unlike the rest of the free world, I didn't get here by accident. And if your parents have you for a reason, then that reason better exist. Because once it's gone, so are you.

Pawnshops may be full of junk, but they're also a breeding ground for stories, if you ask me, not that you did. What happened to make a person trade in the Never Before Worn Diamond Solitaire? Who needed money so badly they'd sell a teddy bear missing an eye? As I walk up to the counter, I wonder if someone will look at the locket I'm about to give up, and ask these same questions.

The man at the cash register has a nose the shape of a turnip, and eyes sunk so deep I can't imagine how he sees well enough to go about his business. "Need something?" he asks.

It's all I can do to not turn around and walk out the door, pretend I've come in by mistake. The only thing that keeps me steady is knowing I am not the first person to stand in front of this counter holding the one item in the world I never thought I'd part with.

"I have something to sell," I tell him.

"Am I supposed to guess what it is?"

"Oh." Swallowing, I pull the locket out of the pocket of my jeans. The heart falls on the glass counter in a pool of its own chain. "It's fourteen-karat gold," I pitch. "Hardly ever worn." This is a lie; until this morning, I haven't taken it off in seven years. My father gave it to me when I was six after the bone marrow harvest, because he said anyone who was giving her sister such a major present deserved one of her own. Seeing it there, on the counter, my neck feels shivery and naked.

The owner puts a loupe up to his eye, which makes it seem almost normal size. "I'll give you twenty."

"Dollars?"

"No, pesos. What did you think?"

"It's worth five times that!" I'm guessing. The owner shrugs. "I'm not the one who needs the money." I pick up the locket, resigned to sealing the deal, and the strangest thing happens—my hand, it just clamps shut like the Jaws of Life. My face goes red with the effort to peel apart my fingers. It takes what seems like an hour for that locket to spill into the owner's outstretched palm. His eyes stay on my face, softer now. "Tell them you lost it," he offers, advice tossed in for free.

If Mr. Webster had decided to put the word freak in his dictionary, Anna Fitzgerald would be the best definition he could give. It's more than just the way I look: refugee-skinny with absolutely no chest to speak of, hair the color of dirt, connect-the-dot freckles on my cheeks that, let me tell you, do not fade with lemon juice or sunscreen or even, sadly, sandpaper. No, God was obviously in some kind of mood on my birthday, because he added to this fabulous physical combination the bigger picture—the household into which I was born.

My parents tried to make things normal, but that's a relative term. The truth is, I was never really a kid. To be honest, neither were Kate and Jesse. I guess maybe my brother had his moment in the sun for the four years he was alive before Kate got diagnosed, but ever since then, we've been too busy looking over our shoulders to run headlong into growing up. You know how most little kids think they're like cartoon characters—if an anvil drops on their heads they can peel themselves off the sidewalk and keep going? Well, I never once believed that. How could I, when we practically set a place for Death at the dinner table?

Kate has acute promyelocytic leukemia. Actually, that's not quite true—right now she doesn't have it, but it's hibernating under her skin like a bear, until it decides to roar again. She was diagnosed when she was two; she's sixteen now. Molecular relapse and granulocyte and portacath—these words are part of my vocabulary, even though I'll never find them on any SAT. I'm an allogeneic donor—a perfect sibling match. When Kate needs leukocytes or stem cells or bone marrow to fool her body into thinking it's healthy, I'm the one who provides them. Nearly every time Kate's hospitalized, I wind up there, too.