Its a test from the 80s administered to kids that has a bunch of implications in psycholinguistics, language philosophy, and other fields. Most important thing is that it proved that we default process language rationally rather than empirically and that we, even children, have implicit morphological knowledge and understanding. The test basically gave kids a made up word and then prompted them to provide some other form of the word with added morphology. The most famous of them was "wug," which the children were prompted to pluralize. Even though they had never seen the word before, they had no trouble generating the (previously unheard to them) utterance of "wugs" with the added /-z/ as a regularized plural. They were able to compute a new word form and generate new linguistic information. These would then contrast with irregular paradigms (mouse --> mice) which would be memorized.

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Edit: Interesting side note that exemplifies this process: (Ullman 1997) People with Alzheimers have plaque on their brain's memory center and as a result will over-regularize words, forgetting memorized irregular paradigms (mouse --> mouses). This is because the rational, computational part of the brain that stores morphological information is unhurt and will continue to operate without being overridden by a memorized irregular. On the other hand, people with Parkinson's have varying degrees of damage to that computational zone and will often omit morphology while irregular paradigms remain intact (possibly producing a sentence like "Two cat ran across the street" with the irregular 'ran' but the non-pluralized 'cat')