I did not know that yesterday!

Many divers believe a little saliva on the ol' scuba mask keeps your vision fog-free. How well does it work? Like a lot of life's mysteries, it depends on who you ask.

The experts at ScubaGuide.com believe it does. "Saliva works very well as a mask defogger, for reasons that have never been adequately explained." "You can purchase a product at the scuba shop to prevent mask fogging. But this 'industrial spit' will work no better than your own." Interesting, though maybe not entirely accurate. ScubaDiving.com tested the various products and concluded that most (but not all) worked better than spit.

But a ranked assessment of artificial saliva is probably more information than the average person needs. The real question is why does spit work at all? FreeDiver.net explains, "Spit keeps the air on the inside of the mask from condensing on the glass." Masks fog up because the inside is often dirty or dusty. Spit cleans off the dirt, making it much harder for condensation and fog to form. It may not be sanitary, but for most divers it works just fine.
source: ask.yahoo.com