Work backwards from the result you want, and keep asking (and answering) the question “What can trigger this?” You have the final goal of “catapulting an object”, so now you just have to figure “what can trigger the catapult?” Brainstorm that for awhile until you get a list of triggers that seem possible / practical for you (and keep that list, because you may need it again), and then figure, again, what can trigger this trigger? You might end up with a dead end at some point, and need to go back to an alternate idea, which is why I suggest that you retain all of your brainstorm ideas. If you were going to be really fancy, then you should have separate mechanisms to: 1. Load the catapult basket 2. Prime the catapult (put the tension on it to make it ready to deliver) 3. Trigger the catapult But that’s just my own one-minute brainstorm. Have fun with this. My physics classes were never so much fun.