A visual illusion closely related to the horizonal-vertical illusion that occurs when a person is invited to guess the depth of a teacup, relative to a teaspoon lying horizontally on the table, by grasping the teaspoon's handle at the point that would be opposite the top of the cup if the teaspoon were standing vertically in the cup (see illustration). Most people grossly overestimate the depth of the cup relative to the teaspoon.
Teacup illusion. If the teaspoon were standing upright in the teacup, which of the marks on the spoon would be exactly opposite the top of the cup? The answer is 1.