Here's why:

* Asteroids are rocky, irregularly shaped objects that orbit the Sun. They are much smaller than planets and are found mainly in the asteroid belt between Mars and Jupiter.

* Planets are also made of rock (in the case of the inner planets) but they are much larger than asteroids and have a more spherical shape.

* Comets are made primarily of ice and dust, not rock, though they may have a rocky core.

Let me know if you have any other questions!