Today I learned that cigarette filters were designed to spontaneously brown on use to give the illusion that they had trapped a large amount of tar or whatever:

Today I learned that cigarette filters were designed to spontaneously brown on use to give the illusion that they had trapped a large amount of tar or whatever: https://en.wikipedia.org/wiki/Cigarette_filter#Colour_change https://en.wikipedia.org/wiki/Cigarette_filter#Colour_change