Teethe is a verb, and it describes what a baby is doing when its first teeth are pushing up through the gums. You almost always meet it in the continuous form: 'The baby is teething', 'He was teething and cried all night.' It describes a stage, roughly from six months to two or three years, and it carries the whole picture with it — the sore gums, the dribbling, the chewing on everything, the bad nights. So when a parent says 'she's teething', they are not just giving you a dental fact; they are explaining why nobody in that house has slept. Two things about the form. The spelling ends in -the and the sound is voiced, like the 'th' in 'breathe', not the one in 'teeth' — teethe rhymes with breathe. That pairing is a neat one to remember: teeth/teethe works exactly like breath/breathe, one noun and one verb. Also note the fixed phrase 'teething problems' (British) or 'teething troubles' — the small difficulties a new system or product has at the very start. 'The app has some teething problems, but they'll fix them.' That is a very common use, and it is much more likely to show up in your reading than babies are.