When the postman arrived at the Smith home to deliver mail through a slot in the front door, he was surprised to find the Smiths had acquired a vicious dog. The dog was fastened to a tree by a chain just long enough to reach the front door. The dog growled and bared its teeth.
How did the clever postman outwit the dog and reach the door safely?
The postman ran around the tree in circles. The dog followed him until the chain wrapped around the trunk enough times to keep the dog away from the door. The dog was not smart enough to unwind the chain.