At our 18th birthday my twin dared me in front of 200 people to take off my robe. So I did — and told them every scar on my body was from saving her life.

PART 1

Two minutes before, the whole back garden was music and splashing and nearly two hundred people laughing.

Then the microphone screamed with feedback.

Every conversation stopped, and everybody turned towards the pool, where my twin sister was standing with the mic in her hand.

Blythe looked perfect. Neon pink swimsuit, skin gold from three weeks of lying on a lounger, completely at ease with every eye in that garden on her.

She has always been built for attention. It comes to her the way weather comes to a hill.

Inside a few seconds there were phones up everywhere, waiting for whatever she was about to do.

I was standing on the other side of the patio in exactly the same swimsuit.

Nobody could see it. I had a thick white towelling bathrobe tied round me, belt knotted twice, and it was the hottest afternoon of the year.

I was sweating, and it was not the heat.

I already had a fairly good idea what she was going to do, because she had insisted on the matching swimsuits three weeks earlier in a voice I have known since before either of us could talk.

She said my name into the microphone, bright and friendly, as if she were inviting me into a joke.

She said: you've been hiding in that robe all afternoon. You're making everybody uncomfortable.

A few people laughed. Her voice made it easy for them.

And then she pointed across the water at me, and the smile on her face went hard in a way I do not think anybody else in that garden noticed.

She said: we agreed matching swimsuits, remember? Stop hiding. Take off the robe and get in the pool.

She said: or are you embarrassed for everyone to see what you actually look like?

One of her friends — Saffron, who has been her best friend since Year Seven — started clapping, slow and sarcastic.

And within about four seconds the whole garden had joined in.

Take it off. Take it off. Take it off.

To them it was a laugh. It was her twin being shy about a bikini at a party.

Not one of them knew that I had not worn anything that showed my arms, my shoulders or my back in twelve years.

Not at school. Not on holiday. Not in a changing room, not at the doctor's if I could help it, not in my own bedroom with the door open.

Through the glass doors at the back of the house I could see my dad.

He had hold of the door handle, and he had the look of a man about one second away from coming out.

Our eyes met across all of it.

And I gave him the smallest shake of my head.

I did not want him keeping it for me any more.

I want to go back three weeks, because that is where this actually started, and I have told it without this part too many times.

Blythe came into my room with her phone and showed me two swimsuits. Same cut, same pink. She said: matching. For the party. It'll be cute.

I could have said no.

I have said no to her about a thousand things in eighteen years and she has survived every one. I could have said I would wear a sundress. I could have said I was not swimming. I could have said nothing at all and just not bought it.

I said: fine.

And I bought it that afternoon, in my size, and I put it in my drawer, and I did not think about why I had said fine until about the middle of October.

I said fine because I had already decided.

Not consciously. Not in words. But some part of me looked at that swimsuit on her phone and saw a pool and two hundred people and a robe, and said yes to all of it at once.

PART 2

I walked towards her.

Every step was heavier than the one before, and the phones came up as I went, because two hundred teenagers had decided they were about to film the most humiliating moment of my life.

When I was a few feet away, Blythe stepped forward, got hold of my wrist, and pulled me towards the edge of the pool.

And she said it loud enough for every single person in that garden:

Take it off. Show everyone what you've been hiding.

Nobody moved. Somebody put a glass down on the side of the pool.

She did not let go. She leaned in and said:

Go on then. Well?

I looked straight at her.

And I undid the belt.

The robe went down onto the stone.

There was a sound through that whole garden like two hundred people breathing in at once, and then a glass broke somewhere near the water.

The chanting stopped. Every smile went, in about a second.

And for the first time that entire afternoon, nobody was looking at my sister.

They were looking at my arms, from the wrist to the shoulder. At the tops of my shoulders. At my back.

At the scars she had spent years making fun of without ever knowing where they came from.

I took the microphone out of her hand. She let me. I do not think she knew she had let go of it.

And I said:

Every one of these scars is from the night I saved my sister's life.

My mum made a sound behind the glass and put both hands over her face.

My dad let go of the door handle and put his head down.

And Blythe went down onto her knees on the stone, in front of all of them, with her hands flat on the ground.

We were six.

There was a fire in the room we shared, at about eleven o'clock at night, in the house we lived in before this one.

I woke up because the curtains were already going. The whole of the window side was orange and I could not see her bed through it.

I got her out.

I do not remember doing it. I have tried. I remember the heat and I remember the landing and a man in a yellow helmet lifting me off the carpet at the top of the stairs, and I remember that his name was Otis because he told me, and that is all.

He told my mother afterwards that I had been holding Blythe by the back of her pyjamas and would not let go, and that he had to carry us both down together.

That is where the scars are from. My arms, my shoulders, most of my back. Eleven weeks in a specialist unit and four operations before I was nine.

That was the part I said into the microphone.

And I could have stopped there.

That would have been enough. It would have made the point. Two hundred people would have gone home knowing that the girl in the robe was not shy — she had walked through a fire — and my sister would have had to live with having mocked that.

I did not stop there.

I held the microphone, and I looked down at her on her knees, and I said the rest of it.

Blythe started it.

A candle she had taken from the bathroom. Lit with our dad's lighter, which she had found in his coat. Stood on the windowsill, right against the curtain, so she could make shadows on the ceiling with her hands.

She was six years old. She was playing.

And then she fell asleep.

PART 3

Our parents made a decision that week that I have not been able to forgive properly in twelve years.

They told Blythe it was an electrical fault.

In my half of the room. They said I had been messing about near a plug with a hairdryer, and that the wiring in that house was old, and that it could have happened to anybody.

And then they sat me down, at six years old, in a hospital bed with both my arms dressed to the elbow, and asked me never to tell her.

My mother did the asking. My father stood by the window.

She said: she'll carry it her whole life, darling. You don't want that for her, do you?

And I was six, and I loved my sister more than anything on earth, and I said no.

So for twelve years Blythe believed that her sister had set their bedroom on fire by being careless.

And she laughed at the scars.

She called me clumsy at eight. She called me a freak in front of her friends at fourteen, in a changing room, when she saw my back by accident.

At sixteen she told a boy at school that I had done it to myself.

And every single one of those times, she genuinely did not know.

She had been handed a version at six by the two people she trusted most in the world, and she had built her whole idea of me on top of it.

The careless one. The one who got burnt because she did something stupid.

That is the sister she thought she had. And she was cruel to that sister, and that sister did not exist.

My dad came out through the glass doors while I was still standing there with the microphone.

He did not say anything to me. He went and knelt down on the stone next to Blythe and put his arms round her, and she let him, and neither of them said anything for about a minute.

And my mum stayed inside the house, behind the glass, with her hands over her face, and did not come out at all.

That is when I understood that she had known, the whole time, that this afternoon was coming.

She just had not known it would be this one.

There is a thing I remember from when I was about eight and I have never told anybody.

My aunt came to stay. And one evening, late, I came downstairs for water and my mother was on the phone in the kitchen with the door almost closed.

She was telling somebody about the fire. And she was telling it the way she told it to Blythe — the plug, the hairdryer, the old wiring.

But she was not telling it smoothly.

She stopped twice. She went back and started a sentence again. She said the hairdryer and then or it might have been and then the hairdryer again.

She was practising.

I stood on the stairs in my socks and listened to my mother rehearse a lie about me, to somebody I did not know, at ten o'clock at night, two years after the fact.

And I went back up without the water, and I never mentioned it, and I understood something that night that I did not have words for until I was about fifteen.

It was not for Blythe.

A mother who is protecting one child does not need to practise the story two years later on the phone.

A mother who is protecting herself does.

Because if Blythe had lit it, then somebody had left a lighter in a coat, in a hallway, where a six-year-old could reach it. And that somebody had not checked on two six-year-olds before bed that night.

PART 4

Now my part, and there is no honest version of this without it.

At six I did what I was told. That is fair. Nobody expects a six-year-old in a burns unit to argue with her mother.

At twelve I could have told her.

At fifteen, when she said what she said in that changing room.

At sixteen, when a boy I liked looked at me in a corridor after she had told him what she told him.

At seventeen, a hundred times, on a hundred ordinary evenings in the kitchen.

I did not.

The one I think about most is the boy.

His name does not matter. He was in my year and I had liked him for about eighteen months and I had never said so, and one lunchtime he walked past me in a corridor and looked at my arms — I had pushed my sleeves up, just for a second, because it was hot — and then looked away very fast.

Blythe had told him that morning. I found that out later that week.

She did it to herself. Don't ask her about it, it's a whole thing.

I went home that night and I sat on my bed with the door locked and I had the sentence ready.

I actually said it out loud to my own wall. You started it. It was your candle.

I had it in my mouth.

And then I thought about her face when I said it, and I thought about how she would have to walk into school the next morning knowing, and I thought — I am not proud of this — that it would be over.

That once I said it, she would never owe me anything ever again. We would be even. Worse than even.

And I found I did not want to be even with her.

So I did not say it. I went downstairs and ate dinner and passed her the salt.

And it was not to protect her. I have had to be honest about that and it has taken most of a year.

The secret gave me something.

Every time she laughed at me, I knew something that she did not know. I got to stand there and take it and be the good one, quietly, inside my own head.

It is a very powerful thing, knowing that the person being cruel to you owes you their life and does not know it.

It means every single thing they say to you makes them worse and makes you better. You do not even have to do anything. You just have to stand there and keep your mouth shut.

I let my sister be cruel to me for six years because it proved which of us was the better person.

And I did not tell her in a kitchen.

I wore the same swimsuit she did, on purpose, when I could have said no to matching three weeks earlier.

I came to that party in a robe on the hottest afternoon of the year.

I shook my head at my dad through the glass when he was about to come out and stop it.

I chose two hundred phones.

Some part of me wanted her to find out exactly then, exactly there, in front of all of them, on her birthday.

And there are about two hundred videos of my sister on her knees on a patio.

Every one of them went online that night.

PART 5

She did not come out of her room for three days.

When she did, it was about two in the morning, and she came into mine and sat on the end of my bed in the dark, and she did not say anything for a long time.

Then she said: can I see them.

Not I'm sorry. Not why didn't you tell me.

Can I see them.

And I turned the lamp on and pushed my sleeves up, and she looked at them properly for the first time in her life — at my arms, at the ridges, at the patch on my left wrist that is shaped a bit like a map — and she did not say anything at all.

She just looked. For about eleven minutes.

Then she said: I thought you did it.

I said: I know.

She said: I've thought you did it since I was six.

And I said: I know. They told you that. I let you keep thinking it.

She looked at me for a long time after that, and I have never been looked at like that by anybody, and I do not want to be again.

The videos were everywhere by the Monday. There is one with about four million views. My sister is on her knees in all of them and my face is in about half.

I was the one who asked people to take them down.

I wrote a post. I asked everybody who had been at that party to delete what they had. I asked the account with four million to take it down, and they did, eventually, after about two weeks and a great many messages.

A lot of people did not understand why I would want it gone. I was supposed to be the one who won.

I did not win. There is nothing to win. I told the truth, at a party, in the cruellest way I could have chosen, and then I spent a month trying to make it smaller.

My mother and I are not all right. I am not sure we are going to be.

She said, in the October, that she had done it to protect Blythe, and I said: and who protected me, and she did not have an answer, and that is where we are.

My dad has apologised four times. He means it. He stood by a window in a burns unit and let her ask me, and he knows that, and he says it to me about once a month.

Blythe and I are not the same as we were.

We are better, some days, and much worse on others, and she cannot look at my arms without her face going, and I have stopped wearing long sleeves in the house.

Here is the only thing I have.

Everybody who has seen those videos thinks this is a story about a girl who humiliated her bully.

It is not.

It is a story about a secret that was put on a six-year-old by two adults who thought they were being kind — and about the fact that the six-year-old grew up and found out that holding it made her feel good.

That is the part nobody tells you about keeping a secret for somebody's sake.

After enough years it stops being for them.

It becomes the thing that proves you are the better one. And you will let them hurt you, again and again, rather than give it up — because the day you give it up is the day they stop owing you.

If you are holding something like that —

tell them. In a kitchen, on an ordinary evening, with nobody else in the room.