Goggles serve a specific purpose—to help you see in the water. It's an important job—so much so that you may see a couple of paranoid triathletes or open water swimmers with a spare pair of goggles with them at the start line.
Even if your goggles stay firmly on your face, the ill-prepared may have a dilemma once they jump in the water.
Your goggles will start fogging up if you're not careful, and the reason is simple. Your face (inside the goggles) is warmer than the area outside of the goggles—in this case, the water. Condensation creates fog that is a pain in the butt for swimmers in the middle of a hard workout or, even worse, a race.
This is an issue as old as goggles, and stories have been passed around the triathlon and swimming circles on how exactly to combat this nuisance.
Here are some of the more common solutions:
You can budget for a fog-free solution, but your own spit is a cheap and surprisingly effective short-term fix to prevent fogging. Many swimmers, due to the sensitivity of the eyes, would never put anything in their goggles other than a nice loogie.
Rubbing saliva around the inside of your goggles will add a layer of water to the lens that will prevent condensation from forming. And for a while, that will leave your goggles fog-free.