On the walk home from school I was a beaten-down soldier, my shoes feeling heavier with each step and my legs more leaden. I couldn't focus on anything but the swirling of emotions in my mind. So I'm grateful for the familiarity of home. The wafting scent of coffee the second I open the front door tells me that somebody else is home. Probably not Mum; she was working from home yesterday, which means her boss has probably made her come into the office today to make up for it. But Beth won't have gone anywhere.

And sure enough, it's Beth I see in the kitchen. She's changed her clothes since this morning—her outfit of choice is a pink t-shirt with pale yellow bunnies on it ('ironically cutesy' was her description of the t-shirt when she bought it) and straight-legged jeans, with fluffy pink slippers on her feet that look a billion times more comfortable than my leather loafers. Which, thinking about it, I kick off my feet and punt in the general direction of the shoe-rack by the door. Beth smiles when she sees me. "How was your day?"

"Fine, I guess," I shrug. I don't like talking to Beth or Mum about my day, and I never have. It always feels awkward. So my response is the same as it has been every day Beth has ever asked the question.

She presents a steaming mug. "I made you coffee," she says. "And one for me too, because I'm a caffeine whore."

"Thank-you."

"I'm going to take mine up to my bedroom," she says, pointedly. "And I'll probably stay there until dinner. If you need me—perhaps because you want to have a chat—then that's where I'll be."

I nod. "Understood."

Beth disappears, leaving me alone in the kitchen. I suddenly feel the horrible clammy coldness of a wet sock—she's spilled something, and I've gone and stepped in it. Grimacing, I set down my mug and bend down to peel off the sodden sock. By the look of it, it was only water that Beth spilled—a small mercy—but that doesn't make it less uncomfortable. I ball the wet sock up, and—shrugging—remove the other one to match. As I go to throw them in the laundry basket, I figure I might as well put the rest of my uniform to be cleaned too, so I undress then and there in the kitchen. It's not like I need to worry about anyone seeing me, other than my cat Rusty. Beth's in her room, and when Mum goes to the office she's just about never home before 6. And it seems a more economical use of my time than walking up to my room, changing, then walking back down to the basket.

But as I'm dumping my clothes in the basket, I catch a glimpse of my reflection in the window. Every hateful angle illuminated in full colour—the chest I can scarcely bear to look at, that must have changed in some significant but imperceptible way in the last few days because why else would I suddenly hate looking at it? My shoulders, neither the broad, rugged shoulders of a rugby player nor the delicate ones of the girls, and above it a face covered in stubble and too damn angular. I wish my hair was longer—it's barely long enough to touch my ears, and suddenly I'm regretting every single haircut I've ever had.

I see all this and I feel sick. Physically sick. Who would want this body? Who would be happy with it? I feel a million ants crawling beneath my skin, crawling, crawling, and I want to pull the skin off to get at them. I want to cocoon myself and hide away from the world, never to emerge. Never to show my face.

I run upstairs in a fog of distress and throw on whatever ill-fitting and ill-matching clothes will cover me up. Who cares if they go together? Anything's better than having to look at myself.

And then I'm stood in Beth's doorway, all the emotions stuffed down into the whistling, bubbling crockpot of my mind. She looks up from her book, marks the page with a tongue-wetted finger, sets it aside. "Are you alright?"

The crockpot boils over. I explode, every tear I've ever held back suddenly shooting forth as my body rocks and shudders. Before I know it, my legs are growing weak, and then she's there, holding my arms, pulling me into the hug, guiding me to her bed. And she holds me in silence until the tears at last start to subside. Soon there's only the gasping half-breaths left.

"There," Beth says. "Does it feel better to let it all out?"

I nod.

"For a moment I was worried you were about to flood my bedroom. This carpet's not waterproof, you know." She laughs, and I laugh, and oh it feels good to laugh for a second. And then I remember why I was upset in the first place, and my face falls.

Beth holds my gaze for a while. I notice she's got the tissues from somewhere, and her fingers are poised to tear one off. "I was right, wasn't I? This morning?"

"Why did you give me lipstick?"

"Because lipstick's cute, and you deserve to be cute." Beth frowns. "Do you not like the colour? I thought it would suit you."

"Don't worry about the colour," I snap. "Sorry. I shouldn't be angry."

"It's okay. Snap if you want to."

"Can I ask you a question, Beth? I asked you before, but that was ten years ago."

She nods gently. "Ask me anything you want. Any time. We're... siblings."

"Okay." I swallow. "Promise me you won't laugh."

"Just as long as you're not about to come out to me as a clown," she quips.

"Beth, why am I a boy?"

She sits back on her bed, resting her head on the headboard. "You remember, then. Our conversation."

"You said it's because I have a penis," I say, wincing at the word. "But why? What did I do to deserve that?"

"Stay there." Beth climbs off her bed, padding barefoot on her carpeted floor, and opens her wardrobe. With a crash, she pulls out a white plastic tote-box with a clear lid; inside, I recognise the survivors from her legion of childhood toys. Stuffed animals and dolls, cheap art supplies, Lego—Beth had the Belville, with dolls and fairies and plenty of pink, and I always wanted to play with hers rather than my own. From this graveyard of childhood memories she draws out a little bear, well-worn and brown with off-white feet. And I remember Mary-Mary, the bear that guarded Beth's bed at night. She brings Mary-Mary over and puts her on the bed between us.

I frown. "What's Mary-Mary got to do with why I'm a boy?"

Beth strokes the bear's head. "Is Mary-Mary a boy or a girl?"

"She's a girl," I say, immediately. What a silly question—did Beth think I'd forget that?

Beth nods, apparently satisfied. "And why is Mary-Mary a girl? She doesn't have boy parts or girl parts, because she's a stuffed bear. And she doesn't wear clothes, so it's not the way she's dressed."

"You always said she was a girl."

"I did," says Beth. "Mary-Mary is a girl because I say she's a girl. And because she was my best friend when I was small, and part of me, in a way she's a girl because she says she's a girl."

"But she's a bear," I say. "A toy. Toys don't have gender like we do."

"What about me, then?" Beth asks. "Am I a boy or a girl?"

"You're a girl, Beth," I say, nervously laughing. "Obviously."

She looks at me with a piercing stare. "Why?"

"What do you mean, why? Because you have the parts a girl has." I'm too embarrassed to name them—discussing femininity has always made me viciously uncomfortable.

"I have," Beth agrees. "But you've never seen them. So how do you know they're there? How do you know I'm not just pretending?"

I shake my head. "I don't."

"Exactly. So you're just trusting me."

"You act like a girl," I point out. "You dress like a girl. You wear pretty dresses and skirts and tops, you have your nails painted and I'm fairly sure you wear makeup."

Beth nods thoughtfully. "That's true," she says. "But that doesn't make me a girl, does it? I've got six pairs of jeans in my wardrobe. I'm wearing jeans right now, in fact. If I took a t-shirt and a hoodie from your room to wear tomorrow, would I not be a girl any more? The nail polish comes off easily—and I'm not even really wearing makeup today, only a bit of primer that I bet you can't even see."

"No, Beth," I tell her. "You'd still be a girl. Even if you wore my clothes. Come on, even I know that it's not clothes that decide someone's gender."

"Then what? We've already decided that since you've never seen me naked, you can't be sure what parts I have. You said yourself that I'm a girl because I act 'girly'—really misogynistic, by the way, you should work on that. So if it's not my body or my clothes, what is it that makes you so sure I'm a girl?"

This feels more and more like a gotcha question. A trick. A prank. "Beth, I'm really confused," I tell her. "I'm sure you've said yourself that you're a girl."

She beams. "Bingo. Got there eventually. I said. I'm a girl because I say I'm a girl. The rest is just window dressing."

"None of that helps me, though," I tell her. "I don't want to know why you're a girl. I want to know why I'm a boy."

Beth looks at me, and her eyes narrow. "Are you?"

"What?"

"Are you a boy?"

"Beth, what do you mean? Look at me."

"I'm looking at you," she says. "But that doesn't tell me if you're a boy or not."

"I'm fifteen years old," I tell her. "And I'll be sixteen soon. I think I'd know if I wasn't a boy."

Beth nods. "You'd think," she says softly. "But Harry"—I wilt a bit hearing my name—"if you're so sure you're a boy, why is it that you haven't answered my question yet? You keep dancing around it, but it's a yes or no question, so I'm going to ask again. Yes or no this time. Are you a boy?"

"Beth—"

"Yes or no."

I swallow. My throat's gone dry, and no words want to form. I open my mouth—open and close it, like a fish. But no sound comes out. No 'yes'.

Beth nods again. "I thought as much," she says. "Look, gender identity is a really personal thing. Nobody gets to decide what yours is apart from you. Not me, not Mum, not anybody. Some people get lucky—I know I did. I'm a girl, who likes traditionally girly things and has a girl's body. The trifecta. But you don't have to tick all three boxes to be a boy or a girl. It doesn't matter if you have a girl's body or a boy's body, you can still be a girl either way. Shit, we're in the modern world. If you don't like your body, you can change it. The important thing is that whatever your gender is, you are the one who decides. And it doesn't matter if you're a boy or a girl, or neither, or both. There's nothing to be ashamed of."

I'm silent. Her words are washing over me, as refreshing as the lapping of the sea on your feet on a sweltering August day.

"I'll ask you another question now," Beth continues. "A slightly different question. Yes or no again. Are you ready?"

I nod.

"Okay. Are you a girl?"

Wouldn't that be nice. Yes, I want to say—but how would that be fair? It would be a lie, an untruth, making a mockery of Beth, and Jessie, and Olivia, and every other girl in the world. They get to answer 'yes' to that question. They deserve it. I don't. I'd be nothing more than a liar. Sure, a part of me wants to be a girl, but the same is true of every boy. It doesn't mean I am one.

And yet saying 'no' doesn't feel like an option. It would be like closing a door that I've only just realised is open. So instead I say nothing, and just sit there paralysed by indecision until I figure out the only answer I could reasonably give to the question.

"I don't know."