Have you ever noticed a puddle disappear after it rains? The water did not vanish by magic. The sun warmed the puddle, and the water slowly turned into a gas called water vapor that floated up into the air. This change is called evaporation.

Evaporation happens when a liquid gets warm enough to turn into a gas. You cannot see water vapor because it is invisible, but it is all around you in the air. Wet clothes on a line dry because the water in them evaporates into the air.

Condensation is the opposite change. When water vapor in the air touches something cold, it turns back into tiny drops of liquid water. That is why a cold cup gets wet on the outside on a hot day — water vapor from the air cooled down and became drops.

You can watch condensation in the morning too. Dew on the grass and fog on a cold window are both water vapor that turned back into liquid. Even the clouds in the sky are made of tiny water drops that formed by condensation.

Evaporation and condensation work together in a big circle called the water cycle. Water evaporates from oceans and lakes, rises up, and cools into clouds. Then it falls back down as rain, and the whole trip starts over again.

These changes are useful every day. Evaporation helps dry your hair and cool you off when you sweat. Condensation gives us rain to water plants and fill rivers. Understanding them helps us know where water goes and where it comes from.