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Answers: ignoring the obvious "it's only a movie" explanation there is something that puzzles me in back to the future part II. Old biff returns to 1955 to give himself the score book that he will use to bet on and become rich. (we are also constantly reminded of the dangers of meeting oneself by Doc which doesn't seem a problem here)

but when he returns to 2015 shouldn't everything already be different. we only see the change when marty travels back to 1985

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I too thought about this at great length, theres an awesome deleted scene where biff upon coming back to 2015, dies immediately. Apparently sometime in the 1990's Loraine kills Biff, the new altered reality takes this into account upon arriving back-(this was written in the original script and removed later). He actually doesnt die, but disappears. By changing the past you create a new reality, where everything stays the same but what change you incurred by traveling in the past. But by Marty and Doc restoring the events by destroying the almanac, all the events shifts back to normal upon Biff getting back to 2015 (I think the writers ultimately aimed for this theory). Because by 2015, all the events of Doc and Marty going back to 1955 and destroying the almanac have already occurred and taken into account, I know its terribly confusing.

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That is the paradox. If I go back in time to kill my grandfather before my father is born. Then I won't be born. So then I couldn't possible go back to kill him, but then that would mean I would be born.

Anyway, Einstein once said he believed that it was impossible to travel through time in your own 'time' loop. You could only travel into another loop. (Alternate Dimension)

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