Heat gives the particles in ice enough energy to break free of their fixed positions and start sliding past one another. That change from solid to liquid is called melting.

When water vapor cools, its particles slow down and pull together into tiny droplets. This change from a gas back into a liquid is condensation.

Liquid water that warms up enough sends its particles flying off into the air as invisible vapor. That change from liquid to gas is evaporation.

Freezing happens when a liquid loses heat and its particles slow down until they lock into a fixed pattern. The result of that locking is a solid.

Gas particles have so much energy and so little hold on each other that they spread out in every direction. They keep moving until they have filled all the space inside the container.

In a solid, the particles barely move and sit right next to one another in a tight pattern. Being packed closely is what gives a solid its firm, fixed shape.

Adding heat speeds up particles and can give them enough energy to leave their fixed spots or fly off into the air. That is why heating a substance can change its state of matter.

A cold glass cools the water vapor in the air around it, and the slowed particles gather into tiny droplets on the outside of the glass. The change from gas back to liquid is condensation.

Oxygen has no shape, no fixed volume, and spreads out through the whole room you are in. A substance that behaves that way is a gas.

Each pair matches a state or change with what describes it. A solid keeps a definite shape and volume, a liquid keeps its volume but takes the container's shape, a gas has neither a fixed shape nor a fixed volume, and melting is the change from solid to liquid.