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My all-time favorite mechanism for a reveal is a Pepper's Ghost.

http://en.wikipedia.org/wiki/Pepper's_ghost

It is most effective at contrasting two states (high tide vs. low tide,
forest in summer vs. winter, even girl vs. gorilla at a carnival) rather
than a word and an image, but the transition between states is magical.


Another option would be to use a lenticular image (most likely to be
familiar as an old Cracker Jack's prize): 

http://en.wikipedia.org/wiki/Lenticular

Katherine Ziff
Exhibit Developer 
Great Lakes Science Center

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