S.O.
I thought this was just a boy thing! My eldest (girl) never did this. But, both my boys have. I didn't react so calmly. It is definitely a behavior issue and you need to discipline her somehow. One son did it at 5 years old twice, the other (I assume learned from older brother) did it at 2. Both haven't done it after being discplined for it. It could be for attention, or rebelliousness, but I think they just thought it would be funny. Once they found out I didn't think it was so funny, they stopped. They love to catch me off guard and shock me. They think it's the greatest thing.