Cassy adores his ping pong ball race track (ahem, otherwise known as a dry bathtub...) When he's bouncing off the wall weird I encourage him to burn off a little energy here. (Note: despite my frequent complaints he really isn't ALWAYS bouncing off the wall weird. (Frequently, but not constantly.))

Livvy is mildly interested in watching the activity, but has no desire to stand in a tub with ppbs bouncing about her tooties thank-you-very-much.

They do have their own unique interests, don't they?