Mercury is the smallest planet in our solar system and the closest one to the Sun. Because it is so close, it zips around the Sun faster than any other planet, finishing a whole year in just 88 Earth days!
If you look at Mercury, it looks a lot like Earth's Moon. Its surface is covered in deep craters caused by billions of years of space rocks crashing into it, because it has almost no atmosphere to protect it.
Even though it is the closest planet to the Sun, it is not the hottest (that title belongs to Venus). Because it lacks an atmosphere to trap heat, Mercury's temperatures are extreme: boiling hot during the day and freezing cold at night!