 What does the older man know the younger man will do?
There is one fact no sane man can quarrel with ... But some men aren't sane; thus it isn't always so! You will, you know, so why quibble about it? At least, you always have ... or do ... or will. You'll get used to it, of course, but it will take about thirty years. You're wondering whether to give me a drink, as I remember it. And naturally, since we have the same tastes, you can make the same for me as you're having. I'm you thirty years from now, or you're me. It's a real wrench when a man meets himself for the first time. You know, I could try to change things around by telling what happened to me; but he—I—told me what I was going to do, so I might as well do the same. So let's begin when you get up in half an hour and come out with me. Yes, it'll be pretty obvious it must be a time machine. You'll be puzzling over what I'll tell you, and you'll be getting used to the idea that you are the man who makes atomic power practical. You won't exactly believe it, but you'll want to go along. You start to say something, but by then I'm pressing a black button, and everything outside will disappear. Doesn't hurt, and when you pull your arm back, you're still sound and uninjured. "Maybe it's no dimension—or it might be the fifth; if you're going to skip over the so-called fourth without traveling along it, you'd need a fifth. "But...." I let it go, and so do you. If you don't, it's a good way of going crazy. I figured out once that such a universe would need some seven or eight time and space dimensions. Maybe there is no machine, and it's just easier for us to imagine it. When you spend thirty years thinking about it, as I did—and you will—you get further and further from an answer. That means you either carry a small time field with you, or you are catching a small increment of time from the main field. You feel a dankish sort of air replace the stale air, and you breathe easier, though we're in complete darkness, except for the weak light in the machine, which always burns, and a few feet of rough dirty cement floor around.
 exactly as he's been told
 exactly as he's been told  invent the next great invention  change the future  fight with him and try not to go