Sikku

trace every line in one stroke. some shapes will not let you, and you can see why.

a kolam in one line: trace every line without lifting, and learn to spot the shapes that cannot be done

A line through a dot uses two of its lines, so the lines at a dot pair up and an odd dot leaves a loose end. A shape goes in one stroke only with no loose ends, or exactly two: start at one, finish at the other.

Concepts: topology, parity, networks, logic.

An interactive you play with one thumb, on Wundr.