换牙 is a separable verb (离合词) meaning the process of losing baby teeth and growing permanent teeth, which typically occurs in children around ages 6–12. It splits between 换 and 牙 when you add measure words, time expressions, or result complements: e.g., 换了一颗牙 ('lost one baby tooth') or 换完牙 ('finished growing permanent teeth'). You can't put an object after the whole word; the child who is teething comes before the verb (他换牙了 = 'he's losing his baby teeth').