Numberlink is a type of logic puzzle involving finding paths to connect numbers in a grid. The first printed version of Number Link can be credited to Nikoli.
Numberlink is an incredibly simple puzzle concept: draw lines in a grid that connect each pair of numbers. While the rules are easy to understand, the puzzles can be difficult to solve without some experience. And while I’ve wanted to construct more Numberlink puzzles for awhile, Numberlink (and related path puzzles) are the single type I am least confident of constructing without errors. Nothing else comes close. (via: Wired.com)