Gases are the only state of matter that can completely fill their container.

Here's why:

* Gas molecules are far apart and move freely. Unlike solids and liquids, gas molecules have very weak intermolecular forces holding them together. This allows them to spread out and move randomly, taking the shape of their container.

* Gases are compressible. Because the molecules are so far apart, you can squeeze a gas into a smaller space. Solids and liquids are much less compressible.

Let me know if you'd like to explore other unique properties of gases!