Roll your mouse over the image to see the view right after the balls fell on the kid pile below.
We went to two hands-on kids science museums on our trip and both of them had some sort of ball setup. The first one was in Chicago and involved an intricate setup of pneumatic tubes for transporting balls from the front of the room to an elaborate water contraption in the back. Eventually the balls would fall, spin, tumble through the various water guides to end up in a small "stream" that then wound its way to the front of the room where kids started the process all over by pulling balls out of a small lake and putting them in the pneumatic tube.
The second museum, in Cleveland and pictured above, wasn't nearly as intricate or covered as much area, but still a major draw to kids. The goal here was to fill an overhead bucket with balls using a pulley belt with small buckets on it (kid powered), a pneumatic tube, or some other means (many parents pitched in, some kids filled containers and climbed a nearby platform to try throwing balls in). Some kids would inevitably lay under the bucket to await the balls being released, while others (like Zane) would purposely avoid it. In all cases everyone was constantly busy.
Not sure if it was accidental or planned, but in both cases, without any kind of guidance or instruction, the kids took it upon themselves to "run" the machine. There was cooperation, delegation, and even group planning ("no, don't dump it yet!"). Kids would come and go, the noise level was loud and mostly unintelligible, and yet everyone seemed to know what to do and how to contribute. You didn't really see any kids hanging out on the sidelines watching or supervising, they either worked on the system or left.