Here's why:

* Ice: Comets are primarily composed of ice, which is frozen water, but can also include other frozen gases like methane, ammonia, and carbon dioxide.

* Dust: Comets also contain dust particles, which are small pieces of rock and metal.

* Rock: The core of a comet, known as the nucleus, is a mixture of ice and rock.

* Orbits the Sun: Comets orbit the sun in highly elliptical paths.

When a comet gets close to the sun, the ice in it vaporizes, creating a glowing cloud of gas and dust called a coma, and sometimes a tail that can stretch for millions of kilometers.