Kage-e ("shadow pictures") -- a popular form of Edo-period woodblock print -- were appreciated by children and adults and commonly used as party gags. These pictures consist of two parts: a "shadow" image and a "real" image. The shadow image, which typically bears the shape of a common, easily identifiable object, is viewed first. The real image, viewed second, reveals the surprising true identity of the shadow.
Here's a nice example by ukiyoe master Kuniyoshi (ca. 1852). It shows what appear to be the silhouettes of goldfish...
But look again...
It's a flying tanuki crushing a hunter under the weight of its mammoth testicles.
Here are a few more kage-e by Hiroshige (ca. 1842). The shadows cast on shoji doors belong to men in interesting poses.
Uguisu (Japanese bush warbler) on a plum branch
Goose on a rock
[Related: Joge-e: Two-way pictures]