Why Is It?

Why is it that you can take not-so-smallish kids to the dentist's office and they climb up the walls, stand on the chairs, swing on the doors, then on to get a haircut where they roll on the floor collecting hair from the last decade, re-arrange the furniture and engage in hand-to-hand combat in such a manner that granny gets knocked off of her walker, then when you return home they collapse on the couch and lie inert for the remainder of the day?