Mercury is a character from ancient Roman stories called myths. He was their god of merchants, messengers, and travelers. Mercury was said to be very fast and good at delivering messages from one place to another. He was often shown wearing a hat with wings on it, which helped him fly around quickly. People would pray to Mercury when they needed help with business or travel, hoping he would hear their requests and make things go smoothly for them. Some people still talk about Mercury today, even though they don't believe in him as a god. They might use his name when they talk about fast or efficient things, like a speedy athlete or a really efficient machine.