You can already get ink that changes colour depending on the heat, I guess they ain’t got round to applying it to work over a period of time. But maybe they should. This would be handy for all sorts of things, Like just writing a line on anything that you don’t want to go over a certain temperature – and seeing the ink if it has. Lots of applications tbh. nice idea.

Thermochromism - Wikipedia

Link: https://en.wikipedia.org/wiki/Th