Plurals are grammatical markers that communicate that there is more than one of something, like cats or cookies. Children who aren’t using these correctly haven’t yet acquired all the rules of English grammar. Plurals are frequently a target in speech therapy for a few reasons: they can be irregular (mice), they are often omitted all together (I have two dog) or they can be mispronounced (using a /s/ for a /z/ sound at the end of a word), giving therapists a lot to target.