K, so I lied a little bit.

Twice, actually.

Nat and I are not in perfect harmony at all. We’re definitely close, and we definitely spend all of our time together, and we definitely adore each other very much, but there are moments now we’ve almost grown up where our interests and passions divide a teensy bit.

Or – you know – a lot.

It doesn’t stop us being inseparable, obviously. We’re Best Friends because we frequently make each other laugh, so much so that I once made orange juice come out of her nose (on to her mum’s white rug – we stopped laughing pretty shortly afterwards). And also because I remember when she peed on the ballet-room floor, aged six, and she is the only person in the entire world who knows I still have a dinosaur poster taped to the inside of my wardrobe.

But over the last few years, there have definitely been minuscule points where our desires and needs have… conflicted a little bit. Which is why I may have said I was a little bit sicker than I actually felt this morning, which was: not much.

Or at all, actually. I feel great.

And why Nat is a bit snappy with me as we run towards the school coach as fast as my legs will carry me.

“You know,” Nat sighs as she waits for me to catch up for the twelfth time. “I watched that stupid documentary on the Russian Revolution for you last week, and it was about four hundred hours long. The least you can do is participate in an Educational Opportunity to See Textiles from an Intimate and Consumer Perspective with me.”

“Shopping,” I puff, holding my sides together so they don’t fall apart. “It’s called shopping.”

“That’s not what’s written on the leaflet. It’s a school trip: there has to be something educational about it.”

“No,” I huff. “There isn’t.” Nat pauses again so that I can try and catch up. “It’s just shopping.”

To be fair, I think I have a point. We’re going to The Clothes Show Live, in Birmingham. So-called – presumably – because they show clothes to you. Live. In Birmingham. And let you buy them. And take them home with you afterwards.

Which is otherwise known as shopping.

“It’ll be fun,” Nat says from a few metres ahead of me. “They’ve got everything there, Harriet. Everything anyone could possibly ever want.”

“Really?” I say in the most sarcastic voice I can find, considering that I’m now running so fast that my breath is starting to squeak. “Do they have a triceratops skull?”

“…No.”

“Do they have a life-size model of the first airborne plane?”

“…Probably not.”

“And do they have a John Donne manuscript, with little white gloves so that you can actually touch it?”

Nat thinks about it. “I think it’s unlikely they have that,” she admits.

“Then they don’t have everything I want, do they?”

We reach the coach steps and I can barely breathe. I don’t understand it: we’ve both run the same distance, and we’ve both expended the same energy. I’m an entire centimetre shorter than Nat so I have less mass to move, at the same speed (on average). We both have exactly the same amount of PE lessons. And yet – despite the laws of physics – I’m huffing and purple, and Nat’s only slightly glowing and still capable of breathing out of her nose.

Sometimes science makes no sense at all.

Nat starts rapping in a panic on the bus door. We’re late – thanks to my excellent acting skills – and it looks like the class might be about to leave without us. “Harriet,” Nat snaps, turning to look at me as the doors start making sucking noises, as if they’re kissing. “Tsar Nicholas II was overthrown by Lenin in 1917.”

I blink in surprise. “Yes,” I say. “He was.”

“And do you think I want to know that? It’s not even on our exam syllabus. I never had to know that. So now it’s your turn to pick up a few pairs of shoes and make ooh and aah sounds for me because Jo ate prawns and she’s allergic to prawns and she got sick and couldn’t come and I’m not sitting on a bus on my own for five hours. OK?”

Nat takes a deep breath and I look at my hands in shame. I am a selfish, selfish person. I am also a very sparkly person: my hands are covered in gold glitter.

“OK,” I say in a small voice. “I’m sorry, Nat.”

“You’re forgiven.” The coach doors finally slide open. “Now get on this bus and pretend for one little day that you have the teeniest, tiniest smidgen of interest in fashion.”

“All right,” I say, my voice getting even smaller.

Because – in case you haven’t worked this out by now – here’s the key thing that really divides Nat and me:

I don’t.