Magic pen invisible ink - how does it work ?

[Anonymous]

So we're on a long car ride - my 5yo dd is doing one of the lee product magic pen invisible ink books - the kind where colors will show up in a picture. She is asking how it works. I called the maker and they told me "trade secret" and it's "magic". Any other more concrete explanations ?

[Anonymous]

Well the ones that show up a particular color on special paper use something called leuco dye, which is colorless until something in the paper triggers the liquid to change into as color.

It's a chemical reaction. Like when we used lemon juice as invisible ink. When dried on paper, heat makes it turn brown.

[Anonymous]

There is a chemical in the pen that reacts to the chemicals in the paper would be my best guess.

[Anonymous]

And the chemicals in the paper must be slightly different in each section since the same ink turns different parts of the paper different colors.