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.

idk tbh because i'm just getting points.

try