At three in the morning when I can't sleep, the room ticks over in the dark and all I have for company is the rush of words coming up fast like those racehorses you see on television, poor things, and when their hearts give out they are laid on the ground and shot dead behind a blue sheet.

At 3 a.m., I think of hearts. I think of candy hearts and carved-tree hearts and hummingbird hearts. I think of hearts in bodies and the rhythm inside us we don't get to choose.

I lay my hand over mine. There it is.

It beatbeats beatbeatbeats skipsabeatbeatbeat

beatbeatbeats.

A heart is a mystery and not a mystery. It hides under ribs, pumping blood. You can pull it out, hold it in your hand. Squeeze. It wants what it wants. It can be made of gold, glass, stone. It can stop anytime.

People scratch hearts into benches, draw them onto fogged windows, tattoo them on their skin. Believe the story they tell themselves: that hearts are somehow bigger than muscle, that we are something more than an accidental arrangement of molecules, that we are pulled by a force greater than gravity, that love is anything more than a mess of nerve and impulse—

'Biz.'

A whisper.

'Biz.'

In the dark.

'Biz.'

In my room.

I open my eyes, and Dad's sitting on the edge of the bed.

'You need to stop,' he says.

What?  I squint at him. He's blurry.

'The thinking. I can hear it when you breathe.'

Dad's wearing a grey sweatshirt. His hands are folded in his lap. He looks tired.

'You should sleep like you did when you were small,' he says. He looks away, smiles. 'Your tiny fingers, tucked under your chin. There's a photo . . .' Dad trails off.

Yeah, Dad. I've seen it.

'The one of us in hospital, after you were born—'

Yeah. The one just after Mum got her new blood and you fainted and they gave you orange juice. The one where Mum's laughing up at the camera as I sleep in her arms. Yeah. I've seen it.

Dad smiles again. He reaches across to touch me, but of course he can't.

That photo has been on every fridge door in every house I've ever lived in. It sits under a plumbing company magnet and beside a clip holding year-old receipts Mum can't seem to throw away.

The photo was taken an hour after I came bulleting out of Mum so fast she had to have a transfusion. In the picture, I look like a slug and Dad looks flattened, like he's seen a car accident. But Mum's face is bright, open, happy.

All the other photos are in albums on our living room bookshelf, next to the non-working fireplace. The albums hold every picture of me Dad ever took until he died, and all the ones of me Mum took until smartphones came along and she stopped printing me onto paper. I'm now partly inside a frozen computer Mum keeps meaning to get fixed, and on an overcrowded iPhone she keeps meaning to download.

And I'm in the photos friends have taken when I've let them and the ones the twins have taken with their eyes since they were babies. I'm in the ocean I walk beside when I skip school and in the clouds where I imagine myself sometimes. And I'm in the look on my friend Grace's face, a second after I kissed her, five seconds before she said she thought of me as a friend.

I blink. Dad's gone again. The room is empty but for me, my bed, my walls, my thoughts, my things.

It's what—four in the morning?

I have a physics test at eight.

My ribs hurt. Behind them, my heart beatbeats beatbeatbeats beatskipsabeat

beatbeat beats.