Penrose triangle

[5] It is featured prominently in the works of artist M. C. Escher, whose earlier depictions of impossible objects partly inspired it.

The tribar/triangle appears to be a solid object, made of three straight beams of square cross-section which meet pairwise at right angles at the vertices of the triangle they form.

Escher's lithograph Waterfall (1961) depicts a watercourse that flows in a zigzag along the long sides of two elongated Penrose triangles, so that it ends up two stories higher than it began.

The resulting waterfall, forming the short sides of both triangles, drives a water wheel.

Escher points out that in order to keep the wheel turning, some water must occasionally be added to compensate for evaporation.

Penrose triangle
A rotating Penrose triangle model to show illusion. At the moment of illusion, there appears to be a pair of purple faces (one partially occluded) joined at right angles, but these are actually parallel faces, and the partially occluded face is internal, not external.
A 3D-printed version of the Reutersvärd Triangle illusion