‘Yes, but supposing you accidentally shot your own grandfather,’ said Yo-less patiently.

‘I wouldn’t. He doesn’t look a bit like Adolf Hitler.’

‘Anyway, you’re not that good a shot,’ said Wobbler. ‘You got kicked out of the Paintball Club, didn’t you?’

‘Only ’cos they were jealous that they hadn’t thought of a paintball hand grenade before I showed them how.’

‘It was a tin of paint, Bigmac. A two-litre tin.’

‘Well, yeah, but in contex’ it was a hand grenade.’

‘They said you might at least have loosened the lid a bit. Sean Stevens needed stitches.’

‘I didn’t mean actually shooting your actual grandfather,’ said Yo-less, loudly. ‘I mean messing things up so maybe you’re not actually born or your time machine never gets invented. Like in that film where the robot is sent back to kill the mother of the boy who’s going to beat the robots when he grows up.’

‘Good one, that,’ said Bigmac, strafing the silent shops with an invisible machine gun.

‘But if he never got born how did they know he’d existed?’ said Yo-less. ‘Didn’t make any sense to me.’

‘How come you’re such an expert?’ said Wobbler.

‘Well, I’ve got three shelves of Star Trek videos,’ said Yo-less.

‘Anorak alert!’

‘Nerd!’

‘Trainspotter!’

‘Anyway,’ said Yo-less, ‘if you changed things, maybe you’d end up not going back in time, and there you would be, back in time, I mean, except you never went in the first place, so you wouldn’t be able to come back on account of not having gone. Or, even if you could get back, you’d get back to another time, like a sort of parallel dimension, because if the thing you changed hadn’t happened then you wouldn’t’ve gone, so you could only come back to somewhere you never went. And there you’d be – stuck.’

They tried to work this out.

‘Huh, you’d have to be mad even to understand time travel,’ said Wobbler eventually.

‘Job opportunity for you there, Johnny,’ said Bigmac.

‘Bigmac,’ said Yo-less, in a warning voice.

‘It’s all right,’ said Johnny. ‘The doctor said I just worry about things too much.’

‘What kind of loony tests did you have?’ said Bigmac. ‘Big needles and electric shocks and that?’

‘No, Bigmac,’ sighed Johnny. ‘They don’t do that. They just ask you questions.’

‘What, like “are you a loony?”’

‘It’d be sound to go a long way back in time,’ said Wobbler. ‘Back to the dinosaurs. No chance of killing your grandad then, unless he’s really old. Dinosaurs’d be all right.’

‘Great!’ said Bigmac. ‘Then I could wipe ’em out with my plasma rifle! Oh, yes!’

‘Yeah,’ said Wobbler, rolling his eyes. ‘That’d explain a lot. Why did the dinosaurs die out sixty-five million years ago? Because Bigmac couldn’t get there any earlier.’

‘But you haven’t got a plasma rifle,’ said Johnny.

‘If Wobbler can have a time machine, then I can have a plasma rifle.’

‘Oh, all right.’

‘And a rocket launcher.’

A time machine, thought Johnny. That would be something. You could get your life exactly as you wanted it. If something nasty turned up, you could just go back and make sure that it didn’t. You could go wherever you wanted and nothing bad would ever have to happen.