Water vapor turns into liquid as it cools due to a process called condensation. When water vapor, which is water in its gaseous state, cools down, the molecules lose energy and begin to move closer together. This transition occurs when the temperature drops to the dew point, which is the temperature at which the air becomes saturated with moisture and can no longer hold all the water vapor in its gaseous form.
As the water vapor molecules lose energy, they slow down, making it easier for them to bond with each other. This bonding process results in the formation of tiny water droplets, effectively transforming the vapor back into liquid water. You can observe this process when you see dew forming on grass in the morning or when water droplets appear on the surface of a cold glass. In essence, the cooling of water vapor decreases the energy needed for these molecules to escape into the air, leading to their transition back to a liquid state.