Molecules don't bounce around "in" something in the same way a ball bounces in a box. They move and interact with each other within a medium, which can be:

* A vacuum: Molecules in a vacuum move freely, colliding with each other and the walls of their container.

* A gas: Molecules in a gas are far apart and move freely, constantly colliding with each other.

* A liquid: Molecules in a liquid are closer together than in a gas, but still move freely. They can slide past each other, but they can't easily escape the liquid.

* A solid: Molecules in a solid are tightly packed together and can only vibrate in place. They don't move freely like in a gas or liquid.

So, it depends on what state of matter we're talking about. It's important to remember that molecules are always moving, even if they are in a solid!