A nonsense word revealed that grammar rules live in the mind, not memory

In 1958, Jean Berko Gleason showed preschoolers a made-up creature labeled a 'wug,' then a second one, and asked them to finish: 'Now there are two ___.' Children who had never heard the word said 'wugs' anyway, proving they had internalized a pluralization rule rather than memorizing words one at a time.