I know a couple of kids, one a boy, the other a girl, both have just turned 3, who do this very weird looking thing. It's hard to explain but I think if any of you have seen it you'll know exactly what I mean. They're lying down on the floor or a sofa, facing down, both hands tucked down in the croach area, but outside the nappy/clothes, and they kind of 'wriggle', make these funny up and down movements. What exactly is going on???