But since it’s a scientifically proven fact that 99.44% of the time no matter what you say or do you cannot create a time paradox situation whereby you change the Past or the Future which you may or may not have previously inhabited*, here goes…
THE NEXT PRESIDENT OF THE UNITED STATES IS GOING TO BE YOU KNOW WHO!
Yeah, that’s right, I said it.
And now you can all quit bugging me about it.
*This is to say, The Future is set in stone just as surely as The Past—at exactly 99.44%–so you generally cannot go back in time and prevent the birth of your worst enemy by killing his Grandfather. Though in theory you could become your worst enemy’s Grandfather by going back in time and knocking up his Grandma. But that’s neither here nor there because the fact that your worst enemy was ever born in the first place demonstrates quite clearly that his Grandfather—whoever he may have been—wasn’t killed before he was able to produce progeny. (99.44% of the time anyway.)
And even if you were to succeed in killing off your worst enemy’s grandfather you would only be destroying the Future you knew, which also happens to be the very same Future that gave rise to you, and that doesn’t really leave you standing in a very good position now does it?
So basically for the most part you can’t ever really change what happened, and the only reason you even think you can change what happens is because you don’t even know what it is yet.
The Omniscients call this “Free Will for Dummies”.