The wug test is a demonstration that normal children do not learn language by imitating their parents. The testee is shown a line drawing of a birdlike creature and told that it is a wug. Then a picture of two of them is shown, and the child is told, "Now there are two of them; there are two ________." Your typical four-year-old will blurt out wugs.
This is repeated with other words like
zat,
sas,
zoop, and
tob. The idea is that a normal person would apply the
rules of grammar to these words despite the fact that they are made up. It shows (especially in children) that we do not just copy each other when we
render language, but rather that we pick up the rules and apply them as we see fit.
People who are language-impaired have trouble doing this. In the above example, a language-impaired adult answered with trouble that the plural of wug was wugness. When presented with sas, she correctly answered that its plural was sasses and upon being told so, she applied the -es suffix to all the other words. She produced "zoop-es" and "tob-ye-es" revealing that she really hadn't grasped the English rule.
Source: The Language Instinct by Steven Pinker (A book I highly recommend.)
This write up was very much inspired by dmd, he introduced me through his writings to cognitive science and linguistics.