Eli5: Kids messless markers

Question

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

Answer 1

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.