You want to mail a letter to a friend. You put an address on it. Then, you add a stamp. The cost of the stamp pays for having the letter taken to your friend. It is easy to send mail. It costs less than a dollar!
A long time ago, people sent letters for free. The people to whom the mail was sent had to pay! People who did not pay could not have their mail! It cost a lot to get letters. It was not fair.
One day, a great man had an idea. He was in charge of the British postal service. Why not use stamps? To send a letter, a person would buy a stamp. A stamp on a letter would show that it had been paid for. Then people would be happy to see the postman coming. They would not have to pay him for their mail! The stamps would not cost much. People could afford to send mail. Everyone would be glad!