I'm probably who Baggy described.
In each mugful of hot milk, there are around 50 - 55 microscopic organisms called Milkies which exist to fulfil but one function in their short (3 - 12 minute) lives; to protect the bottom-feeding Lactotrons from attack by anything trying to infiltrate the colony from above.
You see, as milk cools down, the sensitivity of the cranial bio-armour of the lactotrons increases like you wouldn't munkin believe, and even a dust particle, or, say, a finger, can be enough to terminate their existence, in much the same way that all traces of a sneeze on a window can be eradicated by a good wipe with a hankie.
The Milkies link elbows and, as if singing Runrig's Auld Lang Syne at the end of a mate's wedding, form concentric rings which oscillate in and out so quickly that it seems to you or I that a film of nigh-on impenetrable, eh, film has formed.
Once this occurs, there's only two possible remedies if you want to keep toastie: 1. re-apply heat. 2. Crumble a cookie around the rim of the mug. This draws the Milkies up and out, thus leaving you a window of opportunity in which to continue consumption.