Matrix

drag the two arrows. the whole plane goes with them.

drag the two arrows and watch the whole plane stretch, shear, and fold with them

A matrix is described by where it sends two arrows, right and up; every other point moves with them, stretching or shearing the grid. The determinant is how much a square's area changed, negative if flipped.

Concepts: geometry, symmetry, linear-algebra.

An interactive you play with one thumb, on Wundr.