Depends on bub, my boy would kinda grind his gums together with my nipple in the middle �� Not at all pleasant! I would tell him no, quickly try and get my boob out of his mouth and not put him back on for a bit. A few times, he would grin at me while doing it and I'd give him a smack on his hand as well (this was around 10 to 12 months though, old enough that he knew he shouldn't do it). He soon learnt that he wasn't going to get any boob if he tried to bite me and hasn't done it in months