You can also view previously answered questions in the HSW Question Archive . Thermochromic ink changes color depending on its temperature.
Conductive ink can conduct electricity.
By applying layers of these special inks along with a layer of normal ink using a fairly normal printing press, it is possible to create an extremely inexpensive printed design that changes depending on the amount of electricity it receives. Liquid crystal based thermochromic ink is sensitive to very small changes in temperature, but it is fairly difficult to manufacture. This makes it perfect for use in items like thermometers where you need the sensitivity, but troublesome in an item that needs to be inexpensive and in which a large, abrupt change in temperature will occur. Leucodyes are specially formulated substances that change from a specific color, like blue, to a clear state when subjected to a temperature change of about 5 degrees F or more. Thermochromic inks can be formulated to change color at specific temperatures.