Crayola came out with a series of crayons called Changeables which consisted of 6 crayons that could change color with a color change crayon. Crayola later made OverWriters and Color Switcher markers in the 2000's which did a similar thing with markers. The markers used different ph's to change the color. The crayons probably change in similar ways. The patent is not listed on the back. I will have to find it.
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