Mercury is the closest planet to our Sun. At some parts of its orbit, Mercury is only about 46 million kilometers away from the sun. Because Mercury is so close to the sun, it is very hot there. It is so hot that all the carbon dioxide and oxygen has boiled away, and Mercury has very little air. On Mercury, it's so hot that even some metals like tin melt into liquids, as if they were water. It's about 427 degrees Celsius - more than four times as hot as boiling water.
Venus and Mercury at dawn - Mercury is the one higher up
Mercury is a small planet. It is only about one-third the size of Earth. Mercury never gets closer than about 77.3 million kilometers from Earth. Even though it is so small and far away, you can see it from Earth without using a telescope, if you look just at sunset or sunrise.
Because Mercury is so close to the Sun, it doesn't take very long to go around the Sun. It goes around once every 88 days, so a year on Mercury is only about three months long.
Nobody from Earth has ever been to Mercury, but in 1975 we did send a spaceship there, which took pictures and sent them back to us. Another spaceship reached Mercury in 2011, and sent better pictures back.
Bibliography and further reading about planets: