Catapults, compressed-air systems, levers: each small group is given materials and turns them into one link of a single chain reaction. Any one link is manageable on its own. The real work sits in the handover to the group next to you: where does your construction meet theirs, who agrees that, and how do you test that it actually carries through? The longer the row grows, the more everyone depends on everyone else.
Then comes the moment of truth. One press of the button, and the reaction runs the length of the room, or it stops at the first handover that does not hold. Building your own link is the enjoyable part, but that single run is what the whole room works towards. The format scales a long way too: from a dozen participants to several hundred, the chain simply gets longer.
