Explains how the particles in gases behave

Gas particles are separated far from other gas particles, therefore, they can move freely and can be compressed and can be filled in different shapes of container, unlike solid, where the the particles are close and attracted to each other closely, thus they cannot fill a container or even be compressed because there's no space between the particles.

Also, gas particles move a lot faster than solid or liquid particles, as said above, they have a lot more space to move around.