Comets are small balls of ice and rock. When they move nearer to the Sun's heat, the ice melts and turns to water and gas. This gas becomes the comet's tail. The tails  are visible from Earth. When the comet moves away from the Sun, it gets colder and it loses its tail. Halley's Comet is the most famous comet.

Thousand of asteroids orbit the Sun. Asteroids are made of rock and they never have a tail. Sometimes asteroids hit the Earth. An asteroid hit Earth 66 million years ago and it caused the extinction of the dinosaurs.