Here's why:

* Ice: Comets are largely composed of water ice, along with frozen gases like carbon dioxide, methane, and ammonia.

* Frozen gases: As mentioned above, comets contain frozen gases that turn into vapor when they get close to the sun. This creates the comet's characteristic tail.

* Rocks: Comets also have a rocky component, which can be mixed in with the ice and frozen gases.

Let me know if you have any other questions!