Each page is printed with a wax-resist illustration that’s invisible on the white paper until water hits it; the design surfaces as the wash spreads, and you’re effectively colouring inside lines you can’t see until they show themselves. The trick is good as a gift for a kid because the reveal does the work: there’s no “I can’t draw” step before the painting starts. Adult-friendly too — the surprise of what your wash uncovers is the actual pleasure.