Well, I don't think that it's anything we can control at all, if we're out in public, wearing our clothes, etc. My sister has nipples that, when they're hard, will stick out through her bra, a shirt, a thick sweater-three or four layers of clothes-you can't see mine through my clothes but hers you can. With her they'll stick out if she's cold or horny. Mine will get hard and ache if I'm horny but you can't see 'em through my clothes. If we've been out somewhere together like the mall or someplace where she's been checking out the guys, her nipples may be sticking out through her clothes for a long time, and guys give her boobs a second look when her nipples are hard like that. Sometimes if we can go into one of those restrooms that's got one commode, we'll go in together and lock the door and both unhook our jeans, and reach inside our panties and rub our clits until we come-she'll reach inside her bra and pull and scratch at her nipples because they ache so much from being hard. After she has an orgasm her nipples will go down for a while until she starts to get horny again.