Impossible triangle, c 1954.

UNITED KINGDOM - OCTOBER 18: This is the first model of the 'impossible triangle' one of a series of 'impossible figures' designed and constructed by English geneticist Lionel Penrose in the 1950s. Penrose's discovery of the 'impossible figure' laid the foundation for the scientific study of illusions and investigations into the psychology of human perception. The Dutch artist M C Escher (1898-1972) was heavily influenced by Penrose's discovery, and incorporated an 'impossible triangle' into his well known 'Waterfall' lithograph. (Photo by SSPL/Getty Images)