Okay…you know people always say that travelling back in time, with a “time machine”, wormhole or whatever, would be impossible because if you could go back in time, you might bump into your grandparents before your parents were conceived and somehow your presence prevents one of the conceptions that lead to your parents and ultimately you so that you are never born so are never “in the future” so that you could never use that time machine to travel back and stymie the conception of one of your parents, so you would be born and so could travel back in time and…you get it…it’s a PARADOX. They even made a whole movie franchise from it starring Michael J Fox, after all.
But, here’s the the thing. If you were able to travel back in time and somehow stopped your mother giving birth to you so that you never travelled back in time then, the timeloop might alternatively close so that you’d be stuck in the time before your birth with no time machine…
Anyway, enough of that, here’s an amusing video showing most of the continuity and a few metaphysical bloopers in Back to the Future