Joe: "Wait a second. What are we gonna do? Wait, okay, I know. Here's what we do. We just go to the time machine, then when I get back to the past, I tell her not to do the experiment. Then she won't even be her. That'll work, right?"
Frito: "Uh--"
Joe: "No, no, wait a second. She already is here. So, that must mean I didn't go back in time, right?"
Frito: "Uh--"
Joe: "No, wait, hold on. It just means I haven't done it yet. Okay, so, I go back an tell her not to do the experiment. Then I won't have to do it either because she won't be here. Then I won't have to come back and save her, right?"
Frito: "I--"
Joe: "But then, wait. Why am I still here?"
Frito: "Uh--"
Joe: "How does this time travel work?"