Scientists have discovered time travel. John goes back in time and meets his great-grandfather as a young child. John invites the boy to see the circus. Unfortunately, there's a fire at the circus which kills the boy. What is the paradox?

If John's great-grandfather was killed as a child, John would not have been born. John would not have been able to travel back in time to take his great-grandfather to the circus. So his grandfather would not have been killed. If his grandfather had not been killed, John would have been born and could travel through time.