Many products such as calculators and laptops show information using liquid crystal displays (LCD). Liquid crystals respond to electrical signals.
Thermochromic liquid crystal is a special form of the LCD material, but in this case it changes colour when heated. The film changes colour at 27 degrees centigrade, so will respond to the heat of your hand.
The sheet of thermocolour film is about 6 inches square, and comes with a detailed explanatory leaflet.