Air contains water vapor. When you breath in air, you also breath in water vapor. When the air enters your body, the air heats up because your body is warmer than the air. When the air exits your body (if there is something colder than the air that you exhale in front of your mouth ( in this case a pair of spectacles )), the water vapor condenses on the spectacles so there is water on your spectacles so you can wipe your spectacles. You can actually see this in winter when you exhale because the air surrounding you is colder than the air that you breath out.