The grandfather paradox.

  

    The grandfather paradox is a hypothetical situation that arises when time travel is involved and which seems to contradict the laws of cause and effect. It involves a person traveling back in time and killing their own grandfather before their parent is born, which would prevent their own birth from ever occurring. For example, suppose a person travels back in time and kills their own grandfather before their parent is born. 

  

      This would mean that the person's parent would never be born, and therefore the person themselves would never be born. This creates a paradox because it suggests that the person's own existence is dependent on their own actions in the past, which seems to violate the laws of cause and effect. It is important to note that the grandfather paradox is purely theoretical and is not necessarily indicative of the nature of time travel.