" Sadly the receipt will not survive long enough for Lazlo to read it. It is already fading, as the thermal paper on which it is printed degrades over time. The reason for this is that printing on thermal paper does not mean adding ink to it. Rather, the ink is already encapsulated within the paper, in the form of a so-called leuco dye and an acid. The act of printing requires only a spark to heat up the paper so that the acid and dye react with each other, converting the dye from a transparent state into a dark pigment. It is this cunning paper technology that ensures that cash registers never run out of ink. But over time the pigment reverts to its transparent state and so the ink fades, taking with it the evidence of curry and beer dinners. "

― Mark Miodownik , Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World