So, you know how sometimes we get letters in the mail and they have these little stickers on them with pictures of people? Those stickers are called stamps, and they help pay for the delivery of the letter.
Now, some stamps have pictures of really important people on them - people who did really amazing things that helped make our country better. These people are usually famous for something they did that was really important, or for being really important in the history of the United States.
For example, you might have seen a stamp with a picture of George Washington on it. He was the very first president of the United States! Or maybe you've seen a stamp with a picture of Abraham Lincoln - he was the president during a really important time in our country's history when they were trying to end slavery.
Other famous people who have been on stamps include famous authors like Mark Twain, real-life heroes like Martin Luther King Jr., and even famous movie stars like Marilyn Monroe.
So whenever we put one of these important people's sticker pictures on our letters and mail them, it's like we're saying "thank you" to them for all the great things they did to help make our country the way it is today.