Depends on the boob. I once spent most of a bus ride across town with some girl's boob pressed against me. We struck up a conversation, and she turned slightly toward me, which would normally back her away from me a bit as well, but she managed to stay right next to me, so her boob was against my arm. As we talked, I'd occassionally move my hands as I spoke (which I tend to do anyway, although I was purposefully exaggerating it here) and my arm moving around moved her boob around a bit, but she was either doing it on purpose or didn't care. I couldn't exactly grab her boob, but I could "cop a feel" by moving my arm around and feeling her boob against it. I got a pretty good idea of its composition, density, etc.