I've occasionally thought it would be nice to have a coffee mug with a built-in heater, so it keeps the coffee from getting completely cold. But this is an obviously impractical idea: heating costs serious energy, so you'd have to have a stand to supply the power, which is inconvenient; also there are probably some exciting failure modes.

This morning it occurs to me that there's a much smaller, simpler and more realistic coffee-related gadget that would deliver a similar benefit: a thermometer you could attach to the outside of the mug which would go beep when the temperature dropped to a predefined threshold, signalling ‘last chance to drink up your coffee before it gets unpleasantly cold’. That could much more easily be a standalone device running off a battery.

I suppose you'd also want it to notice if you'd already finished the coffee, and not go beep just because an empty mug had cooled past the threshold. Hmm. Must be something clever you can do about that…