Me: As soon as it's invented, I'm going to travel back in time.
Buddy: Time travel is theoretically impossible.
Me: It is?
Buddy: Sure. Haven't you heard of the grandfather paradox?
Me: No, what's that?
Buddy: What happens if you travel back in time and kill your grandfather?
Buddy: What happens if you kill your grandfather, you know, before your father is born and therefore before you're born.
Me: Why would I do that?
Buddy: See, if you were never born, then you couldn't kill your grandfather, which would be a paradox.
Me: What do you have against my grandfather?
Buddy: You're missing the point.
Me: What's your point? That people who hate their grandfathers can't travel through time?
Buddy: No, nobody can travel in time.
Me: Even people who have no intention of killing their grandfather?
Me: That doesn't make any sense.
Buddy: Okay, forget about your grandfather. Let me think of a way to put this in terms you'll understand. Remember Back to the Future?
Me: Yeah, that was a great movie. I went out and bought an orange vest just like Marty McFly.
Buddy: Remember how he accidentally caused his parents never to meet?
Me: But the orange vest never really caught on.
Buddy: You're not listening. Remember how he had a picture where he started to fade away? And how he had to get his parents to meet and fall in love before he disappeared?
Me: Yeah, that was a great movie.
Buddy: Well if his parents never met, and he disappeared, then how could he travel back to stop them from meeting in the first place? That's a temporal paradox!
Me: Haha - and Biff wound up covered in manure!
Buddy: Are you listening?
Me: Yes. I'll steer clear of my mother and be nice to my grandfather.
Buddy: You're still not getting it. Look, what if you went back in time and, I don't know ... killed Hitler.
Me: Hey that's a good idea.
Buddy: Ok let's say you killed Hitler and so the Nazis and the Holocaust and World War II never happened.
Buddy: Ok if those things never happened, then you would have no reason to go back and kill Hitler.
Me: Even if he was my grandfather?
Me: That's probably why Hitler never had kids. So no one could travel back in time and kill him.
Buddy: I give up.
Me: I'm glad we had this conversation. I'll make sure to leave Hitler alone.
Me: Hey where are you going?
Me: So I guess I'll see you later?