I look over and see Elsa mushing two plums together. ‘Don’t worry,’ I say. ‘I washed them.’

‘Yeah, but –’

‘And she’s my baby.’

Ed’s eyes widen. ‘She’s your baby?’

My cheeks flush. ‘I mean, not mine, obviously, but related to me. We share DNA. Fifty per cent from the mother, our mother. We’ve got different dads.’

Ed looks at me. ‘You mean she’s your sister.’

‘Yes.’

We watch as Elsa abandons the plums and grabs the edge of a table. It tips forward and one of the bags of flour lands next to her, spilling over the grass. Elsa shrieks and plunges both hands into it.

‘Oh!’ I run over and start scooping the flour back into the bag.

Ed helps me. ‘Is she all right with that?’ I look up and see that Elsa’s turned her attention to one of the eggs we were using earlier. She’s trying to put the whole thing in her mouth.

‘Yeah … Or do you think she could choke on it?’

He shakes his head. ‘Too big.’

‘I think it’s one of the hard-boiled ones.’

I put the flour back on the table and try to wipe myself down.

‘So what’s good here?’ Ed asks, looking out of the tent.

‘Everything,’ I say. ‘The Thompson telescope is one of the largest in the world and the floor rises up when you press a button. Oh, and I love the spark disc where you can move electricity with your fingertips.’ I spread my fingers wide to show him what I mean, then cross my arms. I was getting carried away. I need to remember exactly who I’m talking to.

‘Can you show me?’ he asks.

I stare at him, trying to work out if he’s joking, but he just stands there, a blank look on his face, waiting for me to answer.

‘OK,’ I say. ‘I suppose I can tidy up later.’ I walk towards the exit. ‘We can start in the light and colour room.’

‘Aren’t you forgetting something?’ Ed says.

‘Oh, oops!’ I run back and pick Elsa up and she starts slamming the boiled egg into the side of my head. ‘See,’ I say. ‘Definitely hard-boiled.’

That evening an amazing thing happens: I put Elsa in her cot and she falls asleep. No screaming, no tramping over the Downs, no astronomy lessons delivered with a gentle sway. She simply closes her eyes and sleeps.

She must be exhausted from being dragged around the science centre. I showed Ed nearly everything. I thought he’d just look around, but straight away he started freezing crystals and spinning planets. I know he said I was different to how I was at school, but he was different too. It was just small things, like his smile. At school, he does this half-smile, with one eyebrow raised, but when he twisted the dial and heard the squeak of the hidden sound wave he did a proper smile and both sides of his mouth shot up. And I noticed that he bites his nails … I’ve been sitting next to him for years, but I’ve never noticed that before.