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Gleason devised the Wug Test as part of her earliest research (1958), which used nonsense words to gauge children's acquisition of morphological rules‍—‌for example, the "default" rule that most English plurals are formed by adding an /s/, /z/, or /ɪz/ sound depending on the final consonant, e.g. hat–hats, eye–eyes, witch–witches. A child is shown simple pictures of a fanciful creature or activity, with a nonsense name, and prompted to complete a statement about it:

This is a WUG. Now there is another one. There are two of them. There are two ________. Each "target" word was a made-up (but plausible-sounding) pseudoword, so that the child cannot have heard it before. A child who knows that the plural of witch is witches may have heard and memorized that pair, but a child responding that the plural of wug (which the child presumably has never heard) is wugs (/wʌgz/, using the /z/ allomorph since "wug" ends in a voiced consonant) has apparently inferred (perhaps unconsciously) the basic rule for forming plurals.