* Energy is released: When a gas condenses into a liquid, and then a liquid freezes into a solid, the particles slow down and come closer together. This process releases energy into the surroundings.

* Exothermic reactions: Reactions that release energy are called exothermic reactions. The energy released is usually in the form of heat.

Think of it this way:

* Gas: The particles are far apart and moving quickly.

* Solid: The particles are close together and moving slowly. To make the particles slow down and get closer, energy needs to be released.

Examples:

* Water vapor condensing into liquid water on a cold window.

* Steam turning into ice.

* Liquid nitrogen freezing into solid nitrogen.