Thermochromic pads on tube station escalators. For a shirt time, whatever mark is left on the pad will stay on it (depending how hot a persons hand is). Messages, pictures, marks and prints can be left, but will never stay there permanently. Promotes interacting with the environment/other people on the tube.
A brand new fluorescent thermochromic compound. It’s a green crystalline solid (first two pics) what emits a faint red color at room temperature under UV light, but when it’s cooled at about -50 °C, the color of the emission changes from red to blue!
Now we only need to find out that what exactly happens on those crystals when they are chilled.