For your science-minded sister, who loves her mulled wine, consider the heat sensitive ceramic Thermal Battery Mug. It’s designed to look completely black until you pour in liquid over 36°C. Then, by the magic of a science we don’t fully understand (though your sis will), a battery icon appears, charging up and down with green power bars to match the level of your liquid. Science, you have truly outdone yourself.
Holds 300 ml.