probably has no practical value, it's just a confusing game seen from online videos: how to quickly destroy the text on a ticket with an electric splint that straightens your hair.

(video source: self-recording)

tickets are usually printed on thermal paper, which does not rely on extra ink, but is coated with leuco dyes and chromogenic agents. When heated locally, the coating melts and the leuco dye reacts with the chromogenic agent to become a colored molecular form. So if you heat the ticket on a large scale, it will change color.

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the following is an example of the reaction of a leuco dye with a chromogenic agent, where the leuco dye is crystal violet lactone (crystal violet lactone) and the developer is bisphenol A. There are also some other versions of the recipe.

A common choice of chromogenic agents for thermal paper is bisphenol A, so it has also been questioned because of potential health risks, and other chromogenic agents are now used to replace versions of bisphenol A. The part of bisphenol A can be read in the original article, but I won't talk about it here.

there is another interesting related phenomenon in life. I don't know if you have noticed that when serving a dish in a restaurant, waiters often use their fingernails to draw a line in the corresponding place of the ticket. In this way, the thermal paper will also show a line in the local area because of friction.