The Four-eyed Dilemma

Dear Doctor Wizard,
I’m confused. I love to play outside in the snow with my friends, but when I come back inside, my glasses fog up. My friends who don’t wear glasses always pick on me when this happens until I can see again. My teacher says that fog is just water, so what gives? It’s cold and wet outside, but warm and dry inside – so why do my glasses that were perfectly clear outside where it’s wet fog up when I come in where it’s dry?
Signed,
Bespectacled and Bruised

Dear Four-Eyes,
First of all … those aren’t your friends. A friend would help you remove your glasses, then start thumping you.
Now that we’ve got that cleared up, let’s look at the science question.
It’s actually wetter inside than outside – at least there’s more water in the air inside than out.
Now before you go all ‘liar liar, pants on fire’ on me, read on. Even though it feels drier, there’s more water in the air in the nice warm cabin than there is outside in the cold. If there weren’t, your glasses would not fog up. 
Why is that? It’s simple really (at least if you're Doctor Wizard). For the same reason you can dissolve more sugar in warm water than you can in cold water (you knew you could do that, right?), you can ‘dissolve’—or evaporate, to use the correct term—more water in warm air than you can in cold air (in both cases, this is related to the kinetic theory of matter, but if you share that little tidbit with your friends, they’ll beat you up even more. Trust me – I know.  Dr. Wizard was young once too).  So the dry air inside your house holds more water than the wet air outside. You just don't notice it, because water dissolved in air doesn’t feel ‘wet’.
The reason that water forms on your glasses when you come inside is because your glasses are still cold – nearly as cold as they were outside. And because your glasses are still cold, the air that touches the surface of them gets just about as cold. In physics, this little microscopic layer of air on your glasses is called the ‘boundary layer’, and if the glasses are colder than the 'dew point' for the inside air, that means there’s more water in the air than it can hold, and it condenses into dew (which is why they call it the dew point) on your glasses. But as soon as your glasses warm up to room temperature (or at least above the dew point of the inside air), the dew evaporates back into the room air – and your friends better watch out. 
Extra Credit: Read this to your dad – then ask him if he can tell you why frost forms on that mug they pull out of the freezer for him down at the tavern – even though there’s no frost on it when it’s still in the freezer. If he hasn’t had more than a couple, he should be able to figure it out. 

Poindexter Points:  The defrosters in most new cars pull in outside air, and pass it through the air conditioner first, before running it through the heater, and finally over the windshield.  This makes the defroster much quicker and more efficient.  Ten PP's if you can explain why.