When a piece of glass gets moisture on it, you say that it's "fogging up". Some things that often "fog up" in day-to-day life include:

• bathroom mirrors
• people's glasses
• the windshield of your car

"Fog" is a kind of cloud that appears close to the ground and makes it hard to see things. When fog appears, you can say that "it's getting foggy". But you only say "___ is fogging up" for objects that you can see through.

There's also a scientific word for the water that appears on something when the air changes temperature. It's called "condensation". So to explain something "fogging up" in a scientific way, you can say:

When a cool object is exposed to humid air, water vapor in the air turns to liquid water, and condensation accumulates on the object.