The people in M.C. Escher's lithograph, "Ascending and Descending", seem to climb up and up a set of stairs, but keep returning to the same level where they started. This is an optical illusion. Could there be a similar musical illusion? Could someone design a musical scale that always seems to be going up in pitch but also goes all the way around to the beginning? Someone could, and did.
Source: tatoeba (12250071)