Again, don't spoil it if you've seen this before. It's from the same book as the other.
Five people are in a room with no windows, no furniture, etc. The floor is a cement slab. Embedded in the middle of the floor standing vertically is a foot long metal pipe. At the bottom of the pipe is a ping pong ball, with roughly 1mm of clearance all the way around.
Problem: Get the ball out of the pipe without damaging the ball, pipe, or floor. You have the following at your disposal: One carpenter's hammer, a 12" wood ruler, a 6" piece of string, a paper clip, and a refrigerator magnet.