Impossible Triangle
This triangle cannot be built as a real object, only 2D art.
Explanation:
In 1954, Roger Penrose, after attending a lecture by the artist M. C. Escher, rediscovered the impossible triangle and drew it in its most familiar form, which he published in a 1958 article in the British Journal of Psychology, coauthored with his father Lionel.1 Penrose was also unfamiliar with the work of Reutersvärd, Piranesi, and others who had created impossible figures previously. Penrose's impossible triangle, unlike Reutersvärd's earlier version, was drawn in perspective, which added an additional size paradox to the object. In 1961, Escher, inspired by Penrose's version of the impossible triangle (he was sent a copy of the article by the Penroses), incorporated it into his famous lithograph "Waterfall."

