Chromic Materials are the most common 'smart' material there is. These are usually materials that respond to two main stimuli: heat and light. Or, in simplified terms, it reacts whenever there is a change in heat or light. By far and away the most popular chromic type is thermochromic materials. These are materials that respond to heat, hence 'thermo'. This is used in thermometer, where the mercury responds to heat to indicate the temperature of the heat. We have all seen or heard of thermometer strips that change colour when you put them on your forehead, these work by crystals that are half liquid, half solid. These are similar to the crystals one uses on an LCD (Liquid Crystal Display) TV. It is very difficult to explain how these crystals work as such but, in a nutshell, the crystals look a different colour depending on what temperature they are as changes in the temperature make them move closer together or further apart. We have also seen those novelty mugs that change colour if you put hot water in, these are down to something called Leucodyes. Leucodyes are a particular pigment, dye or ink that starts off a particular colour and change to another colour when heat is applied. Leucodyes are organic chemicals that change colour when heat energy makes their molecules shift back and forth. This action, however, can result in very different end results on different materials, but it's cool anyway.