Early on in teaching about systems I often bring out a Slinky. In case you grew up without one, a Slinky is a toy – a long, loose, spring that can be made to bounce up and down, or pour back and forth from hand to hand, or walk itself downstairs. I perch the Slinky on one upturned palm. With the fingers of the other hand, I grasp it from the top, partway down its coils. Then I pull the bottom hand away. The lower end of the Slinky drops, bounces back up again, yo-yos up and down, suspended from my fingers above. “What made the Slinky bounce up and down like that?” I ask students. “Your hand. You took away your hand,” they say. So I pick up the box the Slinky came in and hold it the same way, poised on a flattened palm, held from above by the fingers of the other hand. With as much dramatic flourish as I can muster, I pull the lower hand away. Nothing happens. The box just hangs there, of course.…