Thermochromatic ink is a type of dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 that changes color when temperatures increase or decrease.

Often used in the manufacture of many toys or product packaging, as well as thermometers.

Thermochromatic ink can also turn transparent

Transparency and translucency

In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...

 when heat is applied, an example of this type of thermochromatic ink is found on corners of an examination mark sheet. This proves that the sheet has not been edited or photocopied, and also on certain pizza boxes to show the temperature of the product.