Older children grind their teeth because they are stressed or because they need to get more air. Sometimes the adenoids and the tonsils are obstructing the airway, so the parents of older children should consider a trip to the ENT (Ear, Nose & Throat) doctor if it's a problem. With babies, sometimes it can be stress too. I've read to make sure they have some "relaxing time" before bed, like lullabyes and/or gentle touches/massage from mom, dad or a caregiver, to help them transition from playtime to sleep time. My 11-month-old did it for a few weeks but just stopped on his own. I think the other parents are right when they suspect that they just "discover" it one day while feeling their new teeth. It makes a noise or a different sensation, so they just keep doing it until they get bored with it. I wouldn't make a big deal out of it because as another parent mentioned, it seems the more attention you call to something like that, the more they want to do it. It just depends on the child's personality.