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Water evaporates because of a process called evaporation, which is when liquid water turns into water vapor, a gas. This happens when water molecules gain enough energy to break free from the surface of the liquid. Here's how it works: Energy from Heat: When water is heated, the molecules move faster.
Evaporation is the process that changes liquid water to gaseous water (water vapor). Water moves from the Earth's surface to the atmosphere via evaporation. Evaporation occurs when energy (heat) forces the bonds that hold water molecules together to break. When you're boiling water on the stove, you're adding heat to liquid water.
At room temperature, only a small fraction of water molecules have enough energy to evaporate at any time. Water molecules attract each other through hydrogen bonds. These intermolecular forces hold the liquid together. For a molecule to evaporate, it must break these attractions. The analogy is similar to a rocket escaping Earth's gravity.
Individual molecules can have energy significantly higher or lower than the average. This means that even at 20°C, some water molecules can reach escape velocity to become gas. At the same time, some gas molecules return to liquid, but since liquid contains many more molecules, more leave liquid than enter, causing net evaporation.