Ed sits on the sofa and starts getting paper and textbooks out of his rucksack. The sight of his AQA physics book sitting on our coffee table manages to be both alarming and reassuring at the same time. ‘Where is she?’ he asks.

‘Who?’ I peel Elsa off my foot and sit on the sofa.

‘Your mum?’

‘Oh … She’s at work.’

‘You’ve got everything ready,’ he says, looking at the Post-it notes, highlighters and laptop I’ve got set up on the table.

‘We’ve got a lot to do … Well, I have,’ I say and I start flicking through my cards. ‘I thought my speech was fine, but recently I’ve been having second thoughts … Do you want a drink?’

‘Er –’ he says.

I shake my head. ‘No, I’ll do that later. We should work. Do you want me to tell you what my speech is about?’ Wow. I am talking fast. ‘My grandad says my speech is lacking in passion.’

Ed blinks and I instantly regret my choice of words. ‘OK,’ he says, and I perch on the edge of the sofa, feeling slightly dizzy from all my fast talking. As I start talking about NASA’s Ames research centre and radiation and gravity, I feel myself relaxing and when we argue about migrating planets we could almost be sitting in science … I just have to ignore the fact that Pongo is resting his drooling mouth on Ed’s knee and that Elsa is ramming her baby walker into his feet.

‘Why’s it mooing?’ Ed asks, as Elsa slams her hand down on a sheep button.

‘Because babies like animal noises.’

‘But it’s a sheep.’

‘It was broken when we got it,’ I say. I don’t bother telling him that like everything else in our flat, it came from the charity shop where Mum works.

‘So why haven’t you fixed it?’ There’s this note of challenge in his voice. If there is one area of science that Ed and I are really competitive in, it’s electronics. Ms Edgecombe once got us to build the same circuit in front of the class and I beat Ed by one second. I won a Snickers and I’ve got the feeling Ed’s never forgotten it.

I shrug. ‘She doesn’t care.’

‘Meg, your sister is going to grow up thinking that sheep moo. You are wilfully encouraging her ignorance. Have you got a screwdriver?’

‘Of course I’ve got a screwdriver.’ I jump up. ‘I’ll get my toolbox.’

Five minutes later, the back of the walker is off and Ed and I are sitting on the floor surrounded by wires, drills and bolts. We realise we’re missing some parts, so I get Elsa’s Thomas train and her flashing hammer and we take those apart too. At first we’re both quiet, concentrating on the fiddly job we’ve created for ourselves, then I notice Ed’s watching me as I get to work with the soldering iron.

‘You really like doing this, don’t you?’ he says.

His scrutiny makes me look back down at the wires. ‘Yes,’ I say. ‘I love finding out how things work.’