Start doing the girl doggy-style, facing the window, and then have your friend quietly sneak out of the cupboard and trade places with you without her noticing. Then exit the room, walk in front of the window, and wave to her. Bonus points if you're still naked when you do this.
Aw hell who am I kidding. This whole flow of time thing really confizzles me.
"Vietnam never happened. TIME PARADOX!"
Anyhoo, don't go back in time, because you'll only crush a mosquito and destroy Berlin.
For more information on time paradoxes, Play TimeSplitters: Future Perfect. or watch a ST: Voyager episodes when it goes back in time...They have it down.
"Yeah, I knew you'd say that. Take this key, it's for that door. You won't make it through without it."
"If you couldn't make it through, who gave it to you?"
"Just shut up, I gotta go and save us from a big snake"
Basically, a time paradox occurs if you time travel and change the past so that it erases the possibility of you going to the past in the first place. Which means that events will occur as if you never traveled to the past, undoing your changes so you will travel to the past again, and so on ad infinitum.
Some writers think that it can destroy the universe. Or unleash killer flying time monkeys. Well, we haven't invented time travel so we can't test these theories.
- Worse than that - if you travel to the past to kill someone and succeed, then you will have no reason to travel to the past at all! And without you in the past, events will happen as they originally did before you interfered. Time will be caught in a loop!
- Oh no!
Guy 2: Yeah...
Guy 1: A guy from the future came out of no where...
Guy 2: Go on...
Guy 1: ...and nearly killed Sonic.
Guy 2: OH FUCK NO
Guy 1: YEAH, TIME PARADOX!