We upgrade the weight on Nina's nipples while they're playing naked hopcotch; first some metal clothespins (*much* heavier than wooden ones), then small vise grips. It's important that the vise grips be tight enough that they don't fall off. Also, for the sake of Science we experiment with whether they're worse oriented vertically on nipples so they jolt with each hop, or horizontally so that they twist *and* jolt. Totally Science.