this is an example of a visualy impossible object that can easily be constructed. [>>]
The impossible triangle Penrose L.S. and Penrose R. (1958) cannot be seen as an object lying in normal three-dimensional space. It is, however, perfectly possible to make actual three dimensional objects – not mere pictures – which give the same perceptual confusion. Figure 10a shows an actual wooden object which, when viewed from one critical position, gives the same retinal image as the Penrose triangle. It looks just as impossible – but it really exists. In fact, though one knows all about its true shape, it continues to look impossible from the critical viewing position .
Perceptual illusions and brain models, R. L. Gregory Proc. Royal Society B 171 179-296 [>>]