Span

every spot on the page is some red plus some blue. almost.

drag two arrows and watch the one recipe that reaches the dot -- then line them up and lose the page

Every spot on the page is some of one arrow plus some of another, in exactly one way. As the arrows swing together the recipe gets longer, and when they line up only one line is left to reach.

Concepts: linear-algebra, geometry, scaling.

An interactive you play with one thumb, on Wundr.