Ooooh yeah, makes me feel dizzy thinking about doing stuff they do lol. It's a bit the same as when they shake their heads and keep going and going and going when I'd be spewing! Or the standing in the middle of the yard spinning around and around and around and around... well you get my drift. They just love the feelings of some of these things. It's all new to them and what feelings they get from doing them. I don't think there's any imminent or great danger otherwise TBH I don't think they'd keep doing it - like if it was "hurting" them being upside down they wouldn't keep it up.