The classic optical illusion of the impossible triangle was first created by artist Oscar Reutersvärd in the 1930's and has been seen in popular culture over the years in works by graphic artists such as M. C. Escher. Described by Roger Penrose as "impossibility in it's purist form" This simple shape makes for an impossibly cool t-shirt design.
Impossible triangle's less well known cousin, impossible square, is an equally impossible 4 sided construction. Although not as visually impactful as it's 3 sided counterpart, impossible square does have the spectacular ability to morph into impossible diamond.
Impossible triangle's distant relative, impossible pentagon, is even less well known, and due to it's rather less striking appearance has not received the fame and attention of it's lesser sided relatives. Pentagon finds solace however, in the fact that it is better off than it's hexagonal buddies.