I have small, flat boobs with nipples that kind of protrde out that are almost as sensitve as my clit-my guy will gently suck one into his mouth and gently but firmly chew on it-I moan as loud as if he's licking my clit-it's incredibly intense. If he flicks them hard with his tongue or scratches them with his nails it's almost as good. If he chews on them long enough, the contractions will start in my belly and I'll begin to heave-he won't have to touch me downstairs at all to get me to that point. If I'm really excited during sex my nipples will get rock hard and ache-it's almost like the ache I feel in my clit. Sometimes I just have to grab them and pinch/scratch them when my guy's driving it into me because the ache in my nipples is so strong.