Our friends at Evil Mad Scientist Laboratories contributed several projects to the wonderful The Hungry Scientist Handbook: Electric Birthday Cakes, Edible Origami, and Other DIY Projects for Techies, Tinkerers, and Foodies. Here's their Smart Coaster project.
Smart Coasters are cast-plastic coasters for your drink that light up red when you put a hot drink on top and light up blue with a cold drink. The design is fully analog– no microcontrollers and no programming– and they incorporate a solar cell so that the whole thing is hermetically sealed: waterproof and washable. Complete step-by-step DIY instructions are included for both the electronics and the resin casting.