Jeroen Domburg from SpritesMods was doing some shopping for physics-related stuff on ebay, when he came across something that is called "thermochromic paint". This kind of paint changes its color above a certain temperature. The paint he got was normally dark grey/black colored but would change to white if temperature was above 30 degrees Celsius. After doing some experiments, he decided to make a clock with it. So he coated resistors with this paint, and had enough current to flow within. This heated up the resistors which in turn heated the paint enough to change its color to white. He arranged the resistors in a 7-segment patterns and there it is: the thermochromic paint clock! Although he had some problems when changing digits due to the fact that resistors did not cool down rapidly, the clock works! Check his site for more info.