H.S.
Yup, my little boy started shaking his head around 6 or 7 months (he's 10 months now) and he just loves the reaction it gets! Now he uses it correctly, no for something he doesn't want to do, no if I offer him food that he doesn't want, he shakes his head no all the time. But in the beginning he was just picking up something he'd seen me do, and doing it because it was fun. It takes a lot longer to learn to nod, and shaking a head no is more fun for a kid anyway. My doctor said that they just enjoy doing a "grown up" thing, and they like to communicate with you once they understand what it means. If she shakes her head no when you offer her something, treat the head shake like she's really telling you no and she'll quickly learn what the head movement means.