Suppose you encounter something that looks like a rattlesnake. One possible explanation is that it is a rattlesnake. Another is that it mimics a rattlesnake. Mimicry can arise through evolution (other snakes mimic rattlesnakes to discourage predators) or through human design (rubber rattlesnakes). Normally, it's reasonable to suppose that things are what they appear to be. But this default assumption can be defeated -- for example, if there's reason to suspect sufficiently frequent mimics.