A made-up creature called a 'wug' proved kids learn grammar rules, not just words

In 1958, Jean Berko Gleason showed preschoolers a drawing of an invented creature, a 'wug,' then a picture of two, prompting 'Now there are two ___.' Children who had never seen the word before still said 'wugs,' proving they had internalised the abstract rule for plurals rather than just memorised which real words to inflect.

— Jean Berko Gleason, The Child's Learning of English Morphology — Word, vol. 14, 1958