All You Zombies
by Robert A. Heinlein
Time travel so far is impossible. This is exemplified by the famous grandfather paradox. If you were to build a time machine it would be possible for you to travel back in time, meet your grandfather and kill him before he has any kids. With him dead, your parents would not be born and you could not have been born. Without you, the time machine would not have been built, so no one could have gone back to kill him in the first place - the paradox.
The craziest (greatest?) of the time travel paradoxes was cooked up by Robert Heinlein in his classic short story “All You Zombies.”
A baby girl is mysteriously dropped off at an orphanage in Cleveland in 1945. “Jane” grows up lonely and dejected, not knowing who her parents are, until one day in 1963 she is attracted to a drifter. She falls in love with him. But just when things are finally looking up for Jane, a series of disasters strike. First, she becomes pregnant by the drifter, who then disappears. Second, during the complicated delivery, doctors find that Jane has both sets of sex organs, and to save her life, they are forced to surgically convert “her” to a “him.” [click to continue...]