Here's why:

* Composition: Comets are primarily composed of ice (frozen water, carbon dioxide, methane, ammonia) mixed with dust and rock particles. This is very similar to a snowball, but with the addition of rocky material.

* Structure: Comets have a nucleus, which is the solid, icy core. This nucleus is surrounded by a coma, a cloud of gas and dust that forms when the ice sublimates (turns directly from solid to gas) as the comet approaches the Sun.

* Tail: The comet's tail, composed of dust and gas, forms as the coma is pushed away from the nucleus by solar radiation pressure and the solar wind. This tail, stretching millions of kilometers, is reminiscent of the trail left behind by a snowball as it melts and rolls.

While the "dirty snowball" model is a good starting point, it's important to remember that comets are complex objects with a variety of different structures and compositions. They also evolve as they orbit the Sun, losing mass and changing shape over time.