Is teeth's a word? Would you say my teeth's enamel is coming off, or my tooth enamel is coming off?
closed as off-topic by anongoodnurse, Kris, Robusto, aedia λ, Kristina Lopez Feb 5 '14 at 19:09
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Teeth is the plural. Tooth is the singular.
Teeth's is the possessive plural and Tooth's the singular possessive. This is one way to form the genitive, the other being through of ("my tooth's enamel", "the enamel of my tooth").
Either way it would be awkward. You have correctly noted that the plural of tooth is teeth. As such it should have been
*My teeth' s enamel …
However, we do not say tooth's enamel or teeth's enamel. Instead, we say
The enamel on my teeth …