Maybe something like a magic mug: https://en.wikipedia.org/wiki/Magic_mug https://en.wikipedia.org/wiki/Magic_mug could be made with ink/paint changes that sh

Maybe something like a magic mug: https://en.wikipedia.org/wiki/Magic_mug https://en.wikipedia.org/wiki/Magic_mug could be made with ink/paint changes that show (very approximately) the temperature (I think the ones around have only on/off states).