Before the invention of the postage stamp, it was difficult to send a letter to another country. The sender paid for the letter to travel in his or her own country. Then the person in the other country paid for that part of the trip. If the letter crossed several countries, the problem was worse.
Rowland Hill, a British teacher, had the idea of a postage stamp with glue on the back. The British post office made the first stamps in 1840. They were the Penny Black and the Twopence Blue. A person bought a stamp and put it on a letter. The post office delivered the letter. When people received letters, they didn't have to pay anything. The letters were prepaid.
Postage stamps became popular in Great Britain immediately.
Other countries started making their own postage stamps very quickly.
There were still problems with -international mail. Some countries did not want to accept letters with stamps from another country. Finally, in 1874 a German organized the Universal Postal System. Each country in the UPS agree to accept letters with prepaid postage from the other members. Today the offices of the UPS are in Switzerland, almost every country in the world is a member of this organization. It takes care of any international mail problems.
Today post offices in the every country sell beautiful stamps. Collecting stamps is one of the most popular hobbies in the world, and every stamp collector knows about the Penny Black and the Twopence Blue.