I'm not talking about the stuff they sell in a bottle but a good ol' fashioned loogie. I can't remember when or where I first saw someone spitting into a mask so I don't know how I learned to do it. When I dive from a boat I spit into the mask before every time getting in the water. I have no fogging issues with my masks unless I happen to be trying a new one. Spitting in the mask has become an automatic thing, when I do it I don't expect any benefit from it since the masks don't fog anyways, but I still do it from force of habit.
Today I treated a new mask with a lighter and toothpaste. I put it on and looking in the bathroom mirror gave it the fogging test. It consists of simply blowing air out of my nose into the mask. The mask fogged. "damn!" I thought envisioning a few more lighter and toothpaste proecedures. Then it occured to me to spit in the mask, I don't think I've ever done it when not diving before. So I did and smeared it around some then washed it out with tap water. I put the mask back on and no matter how much I blew from my nose I couldn't get the mask to fog, not even a little. It remained crystal clear. I was truly amazed. So I thought it may be good to remind everybody how well our natural mask defogger works and maybe steer away someone inexperienced from spending money on mask defoggers.