‘What are you doing here?’ my voice is a whisper.

He shrugs, looking amused. ‘Learning about the Cool Cosmos.’

‘And you’ve been here …?’

‘The whole time,’ he says with a smile.

He saw that boy throw the tomato in my eye … He watched me bounce an egg … He heard me say that the cosmos ‘blew me away on a daily basis’!

And then my mind charges through what I’m about to do to my volunteer: rub balloons on their hair and body, then see if an electric shock will pass between our fingers.

With someone who was seven, this would have been funny. But with Ed, it would be … so weird!

Down by my feet, a boy is fiddling with his balloon. He must somehow undo it, because suddenly there’s a hiss of air and the balloon shoots out of his hands and flies round the tent. It flops on the floor by my feet, looking empty and a bit silly.

Wow. I know exactly how that balloon feels.

‘Sorry,’ I say to Ed, ‘but you can’t be the volunteer.’

‘Why not?’ asks his brother.

Suddenly I see a way out. ‘Because I need someone a bit smaller. Someone like you!’

‘No,’ he says, clenching his fists. ‘You said my brother could do it. You promised!’

‘I didn’t promise.’

‘It doesn’t matter, George,’ says Ed, patting his shoulder. ‘If Mega Knickers doesn’t want me to do it, you can take my place.’

‘No way. She said you could do it!’ In slow motion, George’s bottom lip sticks out and it starts to tremble.

‘Bless,’ says a mum on the front row. ‘Let his brother do it, love.’

Are his eyes welling up? I glance at the back of the tent where Maggie is watching me closely. I’m fairly certain she doesn’t want me to make any children cry. ‘Fine,’ I say, nodding. ‘He can do it.’

There’s a scattering of applause and George shouts, ‘Yes!’ and punches the air.

To get through this I’m going to have to stick to the plan and totally ignore who my volunteer is. ‘You’re going to remove some of the electrons from your balloons like this,’ I say, rubbing a balloon on my lab coat, ‘leaving them positively charged. Then when you put the balloon on to Ed’s hair,’ I reach up and balance it on his perfect hair, ‘the balloon will cling to the negatively charged electrons, proving that opposites attract!’

‘So cute!’ says a woman at the back.

‘Ignore her,’ whispers Ed. ‘That’s my mum.’

In a panic, I let go of the balloon and it floats to the ground.

George tugs on my lab coat. ‘You need to rub it on his hair.’

‘Why don’t you do it?’ I say.

‘I’m too small,’ he says with a smile. What’s happened to his wobbling lip?

‘Fine,’ I say. ‘I’ll rub his hair.’ I reach up and start rubbing the balloon all over Ed’s hair, flattening his sticky-up bit at the front. To cover my embarrassment at being chest to chest with Ed King, I rub extra hard. In the four years we’ve been sitting next to each other, we’ve never touched, not even an accidental blazer-brush, and yet here I am giving him a head massage … with a balloon.