I don’t think anyone notices at first.

Jesmond fires again, and again the water hits me. He could hardly miss – I am only about a metre away.

This all happens in a matter of seconds. I know, though, that I have been made slightly visible.

I throw the guitar, shouting ‘Catch, Boydy!’, regardless of what will happen when people hear a voice from nowhere. The hubbub in the hall will be enough, I hope, that no one will be able to tell where my voice is coming from.

Except, at the exact moment I shout, the people nearest notice a strange watery shape appear, where the pistol’s jet has hit me, and a silence falls over that part of the hall.

Aramynta Fell, who is in the next row up, just goes: ‘Oh my God. It’s there again!’

Boydy catches the guitar by its neck, but by now I’m running up the aisle.

A few people are getting out of their seats to follow or simply craning their necks to get a better view of this weird thing. I think they probably think it’s part of Boydy’s act, another illusion.

Behind me I hear Mr Parker shout, ‘Sit down, everybody, sit down! Let the performers perform – show them a modicum of respect!’

But he’s never had a lot of natural authority, and the hall is now in a sort of pack frenzy, with people spilling into the aisle to see what it is that is causing the commotion.

The lighting guys brought up some of the lights in the auditorium as soon as Boydy started his move off the stage, but it’s still not exactly bright.

Meanwhile, Jesmond Knight is firing his water pistol like it’s a proper gun battle, and someone on the other side of the aisle, who has been hit in the crossfire, is returning fire from their own water bottle. Enough of it is hitting me to keep me at least partly visible to the mob advancing up the aisle.

I can hear people saying, ‘What is it?’ and ‘Look – there’s a hand!’, but mostly ‘Oh God!’ and a few other much more extreme expressions. Mainly, though, no one is quite sure what they are seeing, and there’s quite a few people saying things like ‘Awesome!’ and ‘Whoa!’

Riley Colman, who won last year’s physics prize, says loudly, as if he knows everything, ‘Oh, for heaven’s sake – it’s a trick of the light!’ (He’s right, of course: my invisibility is definitely ‘a trick of the light’.)

I’m ahead of the crowd by a good few metres. If I can get outside and run, people won’t be able to see me, I’m pretty certain.

There are double safety doors leading out of the auditorium, with one of those metal bars that you push to open.

I do open it, and I stop dead.

It’s pouring down outside – like, monsoon style. One step into the rain and I’ll be totally visible, a rainy ghost figure.

I turn round, and there are about a dozen people right behind me.

I’m not even sure what they can see. A few drops of water on my face and hair? What does that look like?

Behind the small crowd I can see Boydy, fear contorting his face. At that moment he does something completely brilliant. He screams.

‘Aaaagh! It’s got me! It’s the ghost of Jimi Hendrix and he’s punishing me!’

Brilliant! Instantly, the crowd turns round to see Boydy waving his arms around, pretending to be attacked, and they all laugh.

I take my chance to edge along the back wall of the theatre. I’m wiping water from my face and anywhere else I can, and I think I’m doing OK.

But then Jesmond turns back from Boydy and points at the ground.

‘Footprints!’

I have left a trail of wet footprints, and they lead straight to me. Without waiting, I sprint to the main door of the theatre and burst through it into the school corridor.