"Yeah, 'cos you were never born, so you couldn't go back to kill yourself in the first place."

"But you were already born before you were able to go back?" said Rhys, "So you couldn't disappear, otherwise you wouldn't be around to go back and kill yourself."

"Good thing we can't muck around with Time like that."

"I'll say," said Rhys.

"You could do a lot of good though," I said.

"How?"

"Well, someone could go back from today and tell the people about all the amazing inventions we have in the future. Imagine being able to show Alexander Flemming where the penicillin fungus came from? That would have speeded things up! Or what if you were able to tell the Greeks about steam engines and electricity? Or told people earlier than that?"

"The modem age would have started thousands of years ago! Imagine getting to rockets and space travel four thousand years ago – what would we be doing today!"

"We'd be living in fantastic cities, with everything done by computers and robots, and nobody would be sick!"

"Yeah, we'd completely mess up the history of the world!" said Rhys, "We wouldn't have had all the empires, like Babylon, Persia, Greece, Rome and all that. The world would be one big happy family, with satellites, and electric cars and robots everywhere, and space travel... fantastic!"

"No pyramids in Egypt, no Leonardo da Vinci or Michaelangelo, no Athens ..."

"Why not?" said Rhys.

"Because there'd be no need for them. Everything would be worked out by scientists and computers."

We thought about that for a while, as we ate our lunch. Out on the playground, some kids were starting a game of cricket. The sun was having trouble getting through a thick bank of cloud, and a fresh wind was whipping some scraps of paper across the field.

"Ever wondered why most people don't live to a hundred?" said Rhys, starting his apple.

"No, why?"

"Its not age that gets them, its their body. It breaks