Solid

turn it till every shadow fills its outline.

turn one solid until its three shadows fill the outlines on the floor and two walls; six solids, from a drum to a cube that is not a cube

A shadow answers one question: what does this look like from exactly here. Three lamps ask it along three perpendicular axes, and one solid can answer circle, square and triangle at once. Six solids, each a different set of three answers, ending on three squares that are not a cube.

Concepts: geometry, perspective.

An interactive you play with one thumb, on Wundr.