I nod as if this isn’t news to me at all. ‘It all got a bit much,’ I say. ‘How much do lunches cost?’

‘Two pounds, so that’s twenty pounds your mum owes us. Really, we need it today. We have tried ringing, several times.’

I try to remember exactly how much money Grandad and I have left. Forty pounds? Maybe not even that. ‘And Christine would like to speak to your mum as soon as possible.’ Dawn watches me closely.

‘Fine,’ I say, smiling. ‘She can speak to Mum tomorrow because she’ll be dropping Elsa off.’

Dawn clearly isn’t expecting this. ‘Well … She wanted to speak to her today.’

‘Sorry, Mum’s got an appointment at the hospital.’

‘Well, that’s great!’ says Dawn, and her smile is firmly back in place and so is mine. ‘So pleased she’s back on her feet.’

‘Me too,’ I say, and I give Elsa a wave and walk down the path.

Knowing Mum’s coming back today makes it hard to concentrate at school and during lunchtime in the library I get totally confused by light and atoms and Beer–Lambert, or, to be precise, how I can calculate absorbance from the equation A = 2 − log₁₀ %T. I work at it for twenty minutes, but in the end I give up and go and get some food.

I’m walking out of the canteen with my cheese baguette when I see Ed sitting with Bella and Raj and the rest of their friends. The thought crosses my mind that I could ask him to help me with the equation and then, before I really know what I’m doing, I’m winding my way through the crowded tables until I’m standing behind him.

Bella looks up and sees me standing there. ‘It’s Meg!’ she announces, loudly, and everyone stops talking and turns to look at me, including Ed.

Quickly, I say, ‘Ed, can I ask you something?’

Bella’s eyes grow wide. ‘Oh, wow …’

‘About Beer–Lambert,’ I add, then I thrust my notebook at Ed.

He studies the equation. ‘It’s hard,’ he says, raising his eyebrows, ‘but obviously not too hard for me!’

Bella groans, ‘God, you’re an idiot.’ Then she shuffles along the bench until there’s a small gap between the two of them. ‘If Ed is about to impart his wisdom, you’d better sit down. It’ll take a long time.’ Because everyone is still staring at me, I feel I have no choice but to squeeze in between them.

Ed says, ‘So, this equation lets you calculate absorbance from transmittance data, right?’

I nod.

‘Then I think I get it.’ He takes a pen out of his top pocket and starts to scribble down figures. After watching him for a few seconds, I unwrap my baguette and take a small bite. His friends get bored of watching me and start talking again. Only Bella keeps her eyes glued on me.

‘Do you like this dress?’ she asks, showing me her phone.

I look at the short lacy dress. I have absolutely no opinion about it except that the hexagonal lace looks like a diagram of a molecule. Usually I’d say, ‘It’s nice,’ or something equally vague in the hope that she’d lose interest in me, but I’m sitting in the canteen eating lunch with Ed and Bella, the king and queen of Year Ten, the Jupiter and Saturn of the solar system! If I was brave enough to walk over and talk to Ed, then surely I’m brave enough to tell Bella what I think about a dress?