The Romans believed that gods and goddesses were in charge of everything on Earth. Mercury is named after the messenger for their gods. The Roman Mercury had wings on his helmet and shoes. He could travel very quickly from place to place. The planet Mercury moves quickly around the sun. That is how it got its name.
Mercury is a little bigger than Earth's moon. It is made of heavier materials, like iron. But if you could weigh Mercury and the moon, Mercury would weigh a lot more. Mercury is heavy, but it is small. It would take more than 18 Mercurys to be as big as Earth