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The grandfather paradox is an example of a problem arising from the effect of time travel on causality, the idea that a cause must precede its effect. The paradox suggests that a cause is eliminated by its own effect, thus preventing its own cause and essentially becoming reverse causation. The classic analogy for this, and the one that gives the paradox its name, is a time traveler journeying back in time and killing their own biological grandfather before they can sire children. This means the time traveler could never have come to exist and, as a consequence, can't travel back in time and thus can't kill their own grandfather. That means they then are born and can go back in time, hence the paradox.
cool, right?
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