Heat is energy that makes particles move faster. When enough heat energy is added, particles move fast enough to change a substance from one state to another, like melting ice into water.

Removing heat from water makes its particles slow down until they lock into a solid pattern, forming ice. Water freezes at 0 degrees Celsius.

Raindrops are liquid water that falls from clouds. Inside the clouds, water vapor condenses into tiny liquid droplets, and when they get heavy enough, they fall as rain.

A brick always takes up the same amount of space and keeps its rectangular shape. Having both a definite shape and a definite volume is what makes something a solid.

Fog forms through condensation — warm water vapor in the air cools and turns back into tiny liquid water droplets that float near the ground.

Liquid particles are close together but not locked in place, so they can slide past one another and flow. This is why you can pour water or juice from one cup to another.

Mass tells you how much matter is packed inside an object. A bowling ball has more mass than a tennis ball because it contains more matter.

The heat from the flame gives the solid wax particles enough energy to break apart and become liquid wax. This melting is a change from solid to liquid state.

Carbon dioxide is a gas dissolved in the soda liquid. When you open the bottle, the gas escapes and forms the fizzy bubbles you can see rising to the top.

Each process describes a state change: boiling and melting happen when heat is added, while freezing and condensation happen when heat is removed. The key is knowing which two states are involved in each change.