To explain this, we start with the fact that air contains water in the gas phase, called water vapor. Dew is made of liquid water that has condensed from that water vapor. Dew occurs when objects cool, such as when drops of liquid form on the outside of an ice-cold drink glass. Dew forms when the object, such as the glass, cools down to the dew point temperature. Water molecules in the air continually bombard surfaces, like blades of grass. Some of the molecules stick, forming a very thin film of water.