The Grandfather paradox is an unexplainable situation regarding time travel.
The paradox asks what happens if you were to travel back in time and killed your grandfather . Your grandfather would be dead, meaning you would never have been born, which means you could not of been alive to go back in time and kill your grandfather, but that means your grandfather would be alive and you would be born and so on with seemingly no answer. Some solutions have been made up for this paradox, one of them is that nature would stop you from killing your grandfather. So if you tried to shoot him the gun would just jam or the shoot would miss. Another solution is that if you did kill him you'd be living in a parallel world to your own but your grandfather would still be alive in the other world.