Johnny and the Bomb by Terry Pratchett

don’t go leaping to great big conclusions!’
    In fact he did. He knew he did. All the time. But there was something about the way Kasandra argued that automatically made you take the other side.
    He waved a hand at the trolley.
    â€˜I mean,’ he said, ‘do you really think something could just press the … oh, the handle, or the bags or something, and suddenly it’s hello, Norman the Conqueror?’
    He thumped his hand down on a black bag.
    The world flashed in front of his eyes.
    There was concrete under his feet, but there were no walls. At least, not much in the way of walls. They were one brick high.
    A man cementing the new row looked up very slowly.
    â€˜Blimey,’ he said, ‘how did you get there?’ Then he seemed to get a grip on himself ‘Hey, that concrete’s still— Fred! You come here!’
    A spaniel sitting by the man barked at Johnny and rushed forward, jumping up at Johnny and knocking him back against the trolley.
    There was another flash. It was red and blue and it seemed to Johnny that he was squashed very flat and then pulled out again.
    There were walls, and the shopping trolley was still in the middle of the floor, as was Kasandra, staring at him.
    â€˜You vanished for a moment,’ she said, as if he’d done something wrong. ‘What happened?’
    â€˜I … I don’t know, how should I know?’ said Johnny.
    â€˜Move your feet,’ she said. ‘Very slowly.’
    He did. They met a very slight obstacle, a tiny ridge in the floor. He looked down.
    â€˜Oh, they’re just the footprints in the cement,’ he said. ‘They’ve been … there … ages …’
    Kasandra knelt down to look at the footprints he’d been standing in. They were ingrained with dust and dirt, but she made him take off his trainer and held it upside down by the print.
    It matched exactly.
    â€˜See?’ she said triumphantly. ‘You’re standing in your own footprints.’
    Johnny stepped gingerly aside and looked at the footprints where he’d been standing. There was no doubt they’d been there a long time.
    â€˜Where did you go?’
    â€˜Back in time … I think. There was a man building this place, and a dog.’
    â€˜A dog,’ said Kasandra. Her voice suggested that she would have seen something much more interesting. ‘Oh, well. It’s a start.’
    She shifted the trolley. It was standing in four small grooves in the concrete. They were dirty and oily. They’d been there a long time, too.
    â€˜This,’ said Kirsty, ‘is no ordinary shopping trolley.’
    â€˜It’s got Tesco written on it,’ Johnny pointed out, hopping up and down as he replaced his shoe. ‘ And a squeaky wheel.’
    â€˜Obviously it’s still switched on or something,’ Kirsty went on, ignoring him.
    â€˜And that’s time travel, is it?’ said Johnny. ‘I thought it’d be more exciting. You know – battles and monsters and things? And it’s not much fun if all we can do is— don’t touch it! ’
    Kasandra prodded a bag.
    The air flickered and changed.
    Kasandra looked around her. The garage hadn’t changed in any way. Except—
    â€˜Who repaired your bike?’ she said. Johnny turned. His bike was no longer upside down with a wheel off, but leaning against the wall, both tyres quite full.
    â€˜You see, I notice things,’ said Kasandra. ‘I am remarkably observant. We must have gone into the future, when you’ve mended it.’
    Johnny wasn’t sure. He’d torn three inner tubes already, plus he’d also lost the thingy from the inside of the valve. Probably no time machine could ever go so far into the future that he’d be good at cycle repair.
    â€˜Let’s have a look round,’ said Kirsty. ‘Obviously where we go is controlled by some factor I haven’t discovered yet. If