It started off as a joke: a family friend who was over for Halloween, after quizzing DS20mo for fun about his body parts which he has known in detail for a few months down to his eyelashes, eyelids and earlobes. She laughed and said well, next time she sees him he'll probably know where his xyphoid process is. So after she left, we showed him once for laughs. Well he now shows it off on command, along with clavicle, pelvis, radius, deltoid etc etc...using himself or his Halloween toy skeleton as a model...his dad is an orthopedic surgeon and is just bursting!