Particles in a liquid are close together but not locked in place, which is why a liquid pours so easily. They slip past each other and let the liquid flow.

Heat gives the particles in butter enough energy to break out of their fixed positions and start sliding around. The change from solid to liquid caused by heat is melting.

A freezer pulls heat out of the water, and as the particles slow down they lock into a solid arrangement. That cooling change from liquid to solid is freezing.

Steam is water whose particles have spread out and are zipping around in the air, with no fixed shape or volume. Matter in that spread-out form is in the gas state.

In a solid, the particles are locked into set positions and only vibrate in place. Because the particles are fixed, the solid as a whole keeps a definite shape.

A drying puddle does not vanish — its water particles slowly escape into the air as invisible vapor. That change from liquid to gas without boiling is called evaporation.

Cool grass chills the water vapor in the morning air just enough that it turns back into tiny liquid droplets on the blades. This change from gas to liquid is condensation.

Anything that counts as matter has weight you can measure and fills some amount of room. The room it fills is called space.

Sunlight warms the ice cube and gives its particles enough energy to break free of their fixed spots. As the particles start sliding past each other, the ice melts into liquid water.

Each pair links a process or state to what happens during it. Evaporation turns a liquid into a gas, freezing turns a liquid into a solid, condensation turns a gas back into a liquid, and a gas itself has no definite shape or volume.