Here's why:

* Gases have very weak intermolecular forces. This means the molecules are not strongly attracted to each other and can move freely. They have a lot of empty space between them.

* Liquids have molecules closer together than gases, but they can still move around relatively freely.

* Solids have molecules that are packed tightly together in a fixed arrangement.

Think of it this way: imagine a crowded room (solid), a room with people standing a bit further apart (liquid), and a huge empty stadium (gas).