Here's why:

* Ice: Comets are primarily made of ice, including water ice, frozen gases like methane, ammonia, and carbon dioxide.

* Dust: They also contain a significant amount of dust, which is essentially rock and mineral particles.

* Gas: As a comet approaches the Sun, the ice begins to vaporize, creating a glowing cloud of gas called a coma.

Let me know if you have any other astronomy questions!