I was saying yesterday you never hear jokes anymore so here's one:
An Irishman walks into a bar, orders three pints of Guinness, and sits in the corner, drinking a sip out of each one in turn. When he finishes them, he comes back to the bar and orders three more. The bartender approaches him and asks, "You know, they'll go flat after a while."
The Irishman replies, "You see, I have two brothers. One is in America, the other is in Australia, and since we parted ways, I've not seen either of them. By ordering three beers, I am able to toast them both wherever they are."
The bartender, touched by this gesture, decides not to intervene, and the Irishman becomes a regular in the bar, always ordering three beers and toasting his brothers.
One day, he comes in and orders only two pints. Sensing something amiss, the bartender inquires, "I don't want to intrude on your grief, but I wanted to offer my condolences on your loss. Was it your brother from America or your brother from Australia?"
The Irishman replies, "Oh, no, neither of them has died. I've just given up the drink."
Many versions of fictional time travel that allow backwards travel suggest that the travel cannot happen further back than the initial use of the time machine.
Perhaps my favorite example of this (in terms of narrative, if not logic) is a time machine that is turned on, only to immediately receive a note saying "turn it off RIGHT NOW."
It's also the worst part, every time I think I should invent time travel, I imagine people saying "you invented what? It was always around". No reason to even try.
That's actually what Terminator is going to be about. There was a recent interview with the director, James Cameron, where he mentioned something about time travel. He didn't want to spoil it before it releases though.
I'm excited to see it in theaters when it comes out!