No, comets are not just big rocks that come off the moon. While they do contain rock, they are much more than that. Here's a breakdown:

* Origin: Comets originate from the outer solar system, primarily from the Kuiper Belt and the Oort Cloud. They are not related to the Moon.

* Composition: Comets are made of ice, dust, and frozen gases like methane, ammonia, and carbon dioxide. They're often described as "dirty snowballs."

* Tails: As a comet approaches the sun, the ice and gas vaporize, creating a visible tail that can stretch for millions of kilometers. This tail is made up of dust and gas, and it always points away from the sun due to solar wind pressure.

In contrast, the Moon is a rocky body with a very thin atmosphere. It lacks the ice and frozen gases that are essential to the formation of comets.

So, while both comets and the Moon are celestial objects, they have very different origins and compositions.