Fogging

I tried searching the archives, but I have no idea where to look because I've never seen this type of effect before.
I'm sorry I can't provide an example, but hopefully I can get some pointers:
How would I go about making an icon where it looks like a window fogging up? I would want it to be an animation, and if I had a picture of someone fairly close to the "camera", how could I make it look like thier breath was fogging up the "camera"? Does this make sense? Can anyone help?
Thanks!