This is a solution for problem What Happens if an Irresistible Cannonball Hits an Immovable Post?

The given conditions of the problem are logically contradictory. It is logically impossible that there can exist both an irresistible cannonball and an immovable post. If an irresistible cannonball should exist, then by definition it would knock over any post in its way, hence there couldn’t exist an immovable post. Alternatively, if there existed an immovable post, then by definition, no cannonball could knock it over, hence there could not exist an irresistible cannonball. Thus the existence of an irresistible cannon-ball is in itself not logically contradictory, nor is the existence of an immovable post in itself contradictory; but to assert they both exist is to assert a contradiction.

The situation is not really very different than had I asked you: ‘There are two people, John and Jack. John is taller than Jack and Jack is taller than John. Now, how do you explain that ?’ Your best answer would be,’ Either you are lying, or you are mistaken.’