When water vapor changes to liquid, it is known as condensation. During condensation, water vapor cools down and loses energy. As a result, the vapor molecules slow down and come closer together, eventually forming liquid water. This process often occurs when warm, moist air comes into contact with a cooler surface, such as when water vapor in the atmosphere condenses to form clouds or dew on the ground. Condensation is an important part of the water cycle, as it helps to return water from the atmosphere back to the Earth's surface.