"I'm not sure if I've spelt 'mouth' correctly", wrote a colleague in an email asking me to proofread a short text she'd written. Strange question from a highly educated editor? No, actually a very understandable question. She was using 'mouth' as a verb and, because the 'th' of the verb 'to mouth' is pronounced like the sound 'th' in the, this or either (/ð/is the phonetic symbol), she wondered whether 'mouth' had an 'e' at the end, like similar noun-verb pairs where the 'th' of the noun is pronounced like the 'th' of thing, thin or thirst (phonetic symbol /θ/), but the verb form adds an 'e' and the pronunciation changes to /ð/: teeth/teethe, breath/breathe, cloth/clothe, sheath/sheathe, wreath/wreathe.
Mouth is unusual in that the noun doesn't add an 'e' to become a verb, yet the pronunciation of the 'th' changes. I can't think of any other noun-verb pair that is the same. Smooth and to smooth are both pronounced with a /ð/ sound, but then 'smooth' is an adjective, not a noun. Earth and to earth, berth and to berth and froth and to froth have /θ/ in both, but no extra 'e' in the verb form.