Molecules in a gas move freely, while those in a solid only vibrate in place.

The longer version

Particles in a gas fly about freely, colliding and bouncing off whatever contains them, which is why a gas spreads to fill any container. Liquids sit between the extremes: particles stay in contact but slide past one another, so the substance flows yet keeps a definite volume.

In a solid the particles are locked into place and can only vibrate around fixed positions, giving the material a definite shape. Adding heat makes the vibration stronger, and eventually the arrangement breaks down and the solid melts. That contrast explains why gases are compressible and solids are not.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.