mini-moan: half empty

This has mildly annoyed me for a while but lately it is starting to grate quite massively.
For some reason my mother never drinks to the bottom of her tea cup, she actually leaves about 2 mouthfuls of tea behind.That's particularly annoying because when you pick-up the 'empty' tea cups you end up spilling a mixture of mum slobber and tea on your high quality white DNA branded T-Shirt.
When questioned as to why she leaves tea behind (but never coffee) the answer is "It's cold". That's not even  scientifically possible, the tea she has consumed directly above the 'dregs' would have been colder than the tea beneath it. Why does this same issue not affect Coffee?
Maybe the next time I make her a tea I'll put exactly 2 mouthfuls of liquid into the cup and see if she leaves it untouched because it was 'cold'.