The blivet is an undecidable figure, an optical illusion and an
impossible object. It is an object that appears to have three cylindrical
prongs, but at the base is only divided into two rectangular prisms: the top
surface of the furthest prism becomes the furthest prong while the surface
facing the viewer becomes empty space, and vice versa for the closer prism; the
third, central prong is formed from the empty space between the prisms.
It was known in 1964, and one was shown on the March 1965 cover of Mad magazine (who dubbed it the poiuyt, derived from the last 6 letters on the top row of a typewriter keyboard, right to left), and has appeared numerous times since then. An anonymously-contributed version described as a hole location gauge was printed in the June 1964 issue of Analog Science Fact/Science Fiction, with the comment that "this outrageous piece of draughtsmanship evidently escaped from the Finagle & Diddle Engineering Works".
The artist M.C. Escher was famous for utilizing this object in many of his drawings, lithographs, woodcuts and many such other media.