When you have a post-office box, the postman doesn’t bring letters to you, but you go to the post-office and get your letters and parcels from your box. The box is locked, only you have the key, so the letters and parcels are safe.

One day the headmaster of a school wrote to the post-office and asked for a post-office box for his school. He soon got an answer. It said, “We will give you a post-office box in one month.”