That's not entirely true. While rocks are a major component of many things that strike planets and moons, there are other things that hit them too:

* Dust and Ice: Space is filled with tiny particles of dust and ice. These are common constituents of comets and asteroids.

* Meteors: These are smaller pieces of rock and dust that burn up in the atmosphere.

* Asteroids: These are larger rocky bodies that can cause significant damage if they hit a planet.

* Comets: These are icy bodies that can leave trails of dust and debris as they orbit the Sun.

So, while rocks are a frequent component of space debris, it's not the only thing that hits planets and moons.