The term "Uncle Sam" stands for the United States. He was a real person. He did not wear a hat with stars and a coat with stripes. A long time ago, there was a man named Sam Wilson. He live in Troy, New York. Everyone liked him. They called him Uncle Sam.
The U.S. Army would come to buy food from Sam. Sam would send boxes of food to the Army. He marked the boxes with "U.S." That showed they were for the U.S. Army. A worker saw the letters. He asked what they were for. A man said they were the first letters of Uncle Sam's name. People started saying that anything for the U.S. Army was Uncle Sam's. It is still that way today.
Why did Sam mark the boxes with "U.S."?