The fischertechnik catapult
The catapult is a fun action module that sets off other reactions. In the main model, for instance, it starts a ball rolling, that is eventually catapulted to the goal at the end of the course. In another model, the catapult starts a vehicle, which takes off. Of course, you can also use the catapult to start a chain reaction in your own designs, catapulting objects to hit other objects.