I’m back home before Gram gets in, and change into my school uniform as if I’ve had a normal day.

Normal day? Ha!

There’s a text from Boydy.

That was AWESOME! Can u come round tonite? I’m cooking.

Seems like I’m forgiven then. Plus, I’m super-curious to see what sort of house Boydy lives in.

Tea with Gram that afternoon is a strained affair, mainly because I’m dying to tell her what happened today but – obviously – I can’t.

‘Are you all right, Ethel?’ she asks more than once.

‘Yes, thanks, Gram. I’m just tired.’

That, at least, is the truth. I’m exhausted. Otherwise, though, I feel fine. I keep checking myself over every time I pass a mirror to make sure everything’s in place. It seems incredible, but I really do seem to be able to do this invisibility thing without any ill effects, apart from being sick afterwards.

Best of all, my skin has improved even more. I have a slight rash of spots on my chin, but other than that, the acne has more or less gone. I grin at myself in the mirror. Gram catches me as she passes.

‘Vanity, Ethel, dear. Too long in front of a mirror can be bad for you.’

‘Have you seen, Gram? My skin?’

‘I told you it would clear up in time, darling.’

She’s off out this evening: another meeting, another committee, apparently. At least, that’s what she says. I’m beginning to wonder.

Would you put on make-up for a committee meeting? Well, obviously, no, you wouldn’t if you’re a kid. But an adult? Especially if you’re like Gram and don’t often wear it?

And the thing is, she doesn’t do it in her room. She says goodbye, that she’ll see me later, and gets in her car, where, from my window, I see her adjust her rear-view mirror and apply blusher, lipstick and mascara. Then she drives off.

Months ago, when I first got my smartphone for Christmas, Gram insisted on installing a tracker app – ‘Just for safety, Ethel, darling.’

I’m pretty certain that she doesn’t know that FindU works both ways.

Tonight will be the first time I use it. I’m going to find out where Gram is going.