The Wug Test is a psycholinguistic test created by Jean Berko Gleeson in 1958, designed for gauging linguistic morphological skills in young children. The test shows one of the titular character and says:

THIS IS A WUG.

The test then shows two of the character and says:

NOW THERE IS ANOTHER ONE.
THERE ARE TWO OF THEM.
THERE ARE TWO __________.

The test participants are then asked to fill in the blank. Most often, even young children suggested the correct answer to be "wugs".

The wug itself appears to be a simple sketch of a bird of some kind, with a short bill and a plump body.