Thermocolour film consists of a thin layer of liquid crystals laid on a black background.

If you increase the temperature of the film the liquid crystals move and change the direction they are facing, When they move they alter the wavelength of the light coming from them and therefore change the colour of it.

Thus, as it warms, the colour of the film changes from black to orange to yellow to green to blue.

Films are manufactured for a variety of temperature ranges. Some will change temperature at 25-30C and others not until 100-200C.

What could uses be for a material that changes colour at a certain temperature?

To tell if something is hot?

You could see if a saucepan handle was hot before you picked it up.

To signify the presence of electricity (see right)

Weirdest use so far:

A sticker made from thermocolour film was placed in a urinal in a public toilet. When the sticker was 'peed' on it changed colour and read 'remember to wash you hands'.