In some ambiguous figures, the features of a person or of an animal can suddenly be seen as different features of another individual. Classic examples include the old woman-young woman illusion and the duck-rabbit illusion. Upside-down pictures involve a special case of dual-purpose features in which the reversal is accomplished not mentally, by suddenly "seeing" the alternative, but physically, by turning the picture through 180°.
Ambiguity can also occur, particularly in some geometric drawings, when there is confusion as to which are the front and the back faces of a figure, as the Necker cube, the Thiery figure, and Schröder's reversible staircase.
Related category• ILLUSIONS AND IMPOSSIBLE FIGURES
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