Limit

halve the distance forever. never arrive. still know exactly where.

drag a point into a hole in the curve -- the window zooms with you, and never quite lets you touch it

A limit is what a function is heading toward at a point, whether or not it actually gets there. Drag the point toward the gap in the curve and the view zooms in with you, so the same finger motion buys less and less real distance the closer you get -- that squeeze is not a UI trick, it is the definition: for any gap you name, however small, there is a distance that gets you inside it. Two of the four curves here settle on an exact number no matter which side you approach from. The other two do not, and getting arbitrarily close proves that just as precisely as arriving would have.

Concepts: infinity, geometry.

An interactive you play with one thumb, on Wundr.