Rabbit-duck illusion - An ambiguous figure in which the brain switches between seeing a rabbit and a duck. The duck-rabbit was "originally noted" by American psychologist Joseph Jastrow (Jastrow 1899, p. 312; 1900; see also Brugger 1999). Jastrow used the figure, together with such figures as the Necker cube and Schröder stairs, to point out that perception is not just a product of the stimulus, but also of mental activity (Kihlstrom 2004).
Jastrow's cartoon was based on one originally published in Harper's Weekly (Nov. 19, 1892, p. 1114) which, in turn, was based on an earlier illustration in Fliegende Blätter, a German humor magazine (Oct. 23, 1892, p. 147).
Interestingly, children tested on Easter Sunday are more likely to see the figure as a rabbit, whereas when tested on a Sunday in October, they tend to see it as a duck (Brugger and Brugger 1993, Kihlstrom 2004). Brugger (1999) has provided a comprehensive catalog of duck-rabbit variants, along with data on their ease of reversibility. From 
http://mathworld.wolfram.com/Rabbit-DuckIllusion.html
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