All You Zombies

"All You Zombies" is a science fiction short story by Robert A. Heinlein that was first published in Fantasy and Science Fiction[?] magazine in 1959. The story involves a number of paradoxes involving time travel.

Warning: Wikipedia contains spoilers

The plot of "All You Zombies" concerns a young man who is taken back in time and tricked into impregnating himself (before he had had a sex-change operation), resulting in the ultimate time-travel paradox: a man who is both his own father and mother.