An irregular noun does not follow the usual plural rule. By the age of two or three, children learn to add . Before that, they treat all nouns as irregular. But even if they had been saying "mice," once they learn the plural rule they may begin saying "mouses" instead. Eventually they learn to treat the irregular nouns as special cases, exceptions to the usual rule. Verbs too can be irregular: the past tense of "fly" is "flew" – unless, of course, the word is being used in some novel manner, as in "The batter flied out," where the regular -ed form is nonetheless used. Young children, however, often use the regular form for the central meaning of "fly," as in "The bird flied away."