That's a great question, but it's not quite accurate to say atoms and molecules of gases and liquids "move away from the heat." Here's a better explanation:

* Heat doesn't push things away. Heat is energy, and when you heat something, you're increasing the kinetic energy of its particles (atoms or molecules).

* Increased kinetic energy leads to expansion. The particles move faster and farther apart, causing the material to expand. In the case of liquids, this expansion can lead to boiling, where the liquid changes state into a gas.

* The "empty space" isn't really empty. It's important to remember that even in a gas, there are still particles present, just very spread out. The "empty space" you're referring to is just the space between the particles, which increases when the material is heated.

So, instead of thinking about the particles "moving away from the heat," think about them gaining energy, moving faster, and spreading out.

What fills the space?

The "empty space" isn't actually empty, but is filled with the same particles that were there before, just further apart.

Let me know if you have any more questions about heat and matter!