Eli5: Kids messless markers

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My kid has these colored markers that only work on special paper. If you write on anything else it does nothing. When you write on the special paper it’s clear for a split second then changes to the specific color. How do they work?

In: 3

the special paper is acidic, and the “ink” in the markers is activated by the acidity.

it’s the same basic mechanism behind a litmus test, though in that case the coloring agent is on the paper.