One of my first adventures in making was a trebuchet. The inspiration came from the Nova episode where they built and tested a trebuchet. I watched that with my two older children (10 and 8) My son (10) asked about a month later if we could build one. I said of course. We started with some internet searches to see what others had done. I found some diagrams that looked good but were a little small. We built ours a little bigger (about 3 foot high pivot point).
My wife was not so excited about building a weapon. When she suggested we use it as a science fair project, I agreed and all was ok. A few good days in the shop and we had an operational weapon, I mean science fair project.
The first firing threw a baseball about 60 feet. We quickly lost the baseball in the forest and switched over to rocks. We had some great throws into the forest. Then we discovered really light rocks land right in front of the machine and really big rocks can go straight up. A rock flying straight up 60 feet takes a while to land. That is a confusing few moments followed by a few moments of terror. Except for the science fair experiment we stuck to launching watter balloons after the short experiment with rocks.
For the science fair part I drilled holes in a dozen tennis balls and filled them with varying amounts of silicone caulk. The science was charting the different ranges from the different masses.
Shortly after we built the trebuchet I took the two kids to the Make Faire in San Mateo (2007). They announced the King of Fling competition but we already had plane tickets and the trebuchet was much too large and heavy to check as luggage, but we had a great time watching the other catapults. We drove to the Faire this year (2008) but alas no king of fling competition was held.