Here's why:

* Your breath contains water vapor: The air we exhale is warm and moist, containing a significant amount of water vapor.

* Cold surface: The glass surface is usually cooler than the warm, moist air you exhale.

* Condensation: When the warm, moist air comes into contact with the cooler glass, the water vapor in the air loses energy and changes state from a gas (water vapor) to a liquid (water droplets). This is called condensation.