Here's why:

* Ice and Dirt: Comets are primarily composed of ice (water ice, frozen gases like carbon dioxide and methane) mixed with dust and rock particles.

* Long, Narrow Orbit: Comets have highly elliptical orbits that take them far out into the solar system and then bring them close to the sun. This is why we only see them for a relatively short time as they pass by.

Let me know if you'd like to know more about comets!