I learned in a foster/adopt class that I took that it's totally normal behavior (for boys AND girls...I know it's normal for boys, 'cause all of my boys did it). It's a comfort thing. If it bothers you, you can try teach her to comfort herself in other ways and tell her that it's not polite/lady like and she needs to stop that. If it doesn't bother you for her to do it but just in public (which is what DHR says to do, let them do it, but keep it in private...not necessarily my route, though), then you just tell them that it's only for in the privacy of their own bed/bedroom or in the bathroom and that it's a private thing like picking their nose, and going to the bathroom. But, it's definitely a normal thing, it's just a little more common to find it with boys. My older boys have pretty much grown out of it (4 & 6), but I find my 2yr old still down there from time to time.