Imagine you have a square-shaped room, and inside there is an assassin and a target. And suppose that any shot that the assassin takes can ricochet off the walls of the room, just like a ball on a billiard table. Is it possible to position a finite number of security guards inside the square so that they block every possible shot from the assassin to the target?

Originally released by Knowledgemotion, 2018. Streaming video file encoded with permission for digital streaming by Infobase on February 21, 2020.