Where did something come from, if it was only ever created by itself?
Imagine a time traveler finds a mysterious manuscript, carries it back in time, and gives it to a young scholar, who then spends years writing the exact same manuscript, word for word, which the time traveler later finds and carries back to give to that same scholar. The information exists in an unbroken loop, with every step accounted for, yet it has no actual point of origin at all. Where did it ever actually come from?
Reveal the answer
There's no satisfying answer within the loop itself; that's precisely the paradox. Nothing in the loop violates cause and effect locally, each step causes the next just fine, but the loop as a whole is self-creating with no external origin, clashing with the usual assumption that everything traces back to some ultimate cause. Named after the 'pull yourself up by your own bootstraps' idiom and sometimes called an ontological paradox, it's a staple of time-travel fiction and a genuine open question in any physical theory, such as general relativity, that permits closed timelike curves.
— Robert A. Heinlein, By His Bootstraps — Astounding Science Fiction, 1941