The Wug test, invented by Jean Berko Gleason, tests childrenâs knowledge of grammatical rules; in the case of the âwugâ, children were shown an image of an imaginary creature that the tester claimed was called a âwugâ, when shown an image of multiple of the creatures, the tester asked the child what they were, most children claimed they were âwugsâ applying the grammatical rule that the suffix /s/ is added to words to make them plural.