Grandfather paradox:

A paradox proposed to discount time travel & revel why it violates causality. Say that your grandfather constructs a time machine. Currently, you employ his time machine to go back in time a few decades to a point before he married his wife .You meet him to talk about things, and an argument ensues (presumably he doesn't suppose that you're his grandson/granddaughter), and you kill him accidentally.

If he died before he met your grandmother & never had children, then certainly your parents could never have met (one of them didn't presents!) and could never given birth to you. Additionally, if he didn't live to construct his time machine, what are you doing in the past alive and along with a time machine, if you were never born and it was never constructed?