 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. And don't look at the machine out there too long—until you get used to it, you'll find it's hard on the eyes, trying to follow where the vanes go. 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. I remember just how you feel; I felt the same way when he—that is, of course, I or we—came back to tell me about it, thirty years ago. And you can look at the revenue stamp date, if you still doubt my story. It's a real wrench when a man meets himself for the first time. Some kind of telepathy seems to work between two of the same people. 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. You are completely outside of time and space, as best you can guess how things are. Doesn't hurt, and when you pull your arm back, you're still sound and uninjured. Then you feel silly, because you'll remember that I said you'd ask that. Well, I asked it after I was told, then I came back and told it to you, and I still can't help answering when you speak. "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.
 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