One absent minded ancient philosopher forgot to wind up his only clock in the house. He had no radio, TV, telephone, internet, or any other means for telling time. So he traveled on foot to his friend’s place few miles down the straight desert road. He stayed at his friend’s house for the night and when he came back home, he knew how to set his clock.

How did he know?

An Arab sheikh tells his two sons to race their camels to a distant city to see who will inherit his fortune. The one whose camel is slower wins. After wandering aimlessly for days, the brothers ask a wise man for guidance. Upon receiving the advice, they jump on the camels and race to the city as fast as they can.

What did the wise man say to them?

A Petri dish hosts a healthy colony of bacteria. Once a minute every bacterium divides into two. The colony was founded by a single cell at noon. At exactly 12:43 (43 minutes later) the Petri dish was half full.

At what time will the dish be full?

This logic puzzle was published in Martin Gardner’s column in the Scientific American. You are in a room with no metal objects except for two iron rods. Only one of them is a magnet.

How can you identify which one is a magnet?

How can you throw a ball as hard as you can and have it come back to you, even if it doesn’t bounce off anything? There is nothing attached to it, and no one else catches or throws it back to you.