"Trainspotter!"

"Anyway," said Yoless, "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 have 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 Yoless, 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's 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.