It is a medieval siege weapon!

Well that was the answer - so what was the question?

The question was - what is a trebuchet? We had been asked this numerous times since it first appeared in our summer program. So here is what wikipedia has to say about it.

A trebuchet ( /ˈtrɛbəʃɛt/ treb-ə-shet or /ˌtrɛbjʊˈʃɛt/ treb-ew-shet;[1] French: trébuchet) is a siege engine that was employed in the Middle Ages. It is sometimes called a "counterweight trebuchet" or "counterpoise trebuchet" in order to distinguish it from an earlier weapon that has come to be called the "traction trebuchet", the original version with pulling men instead of a counterweight. Man-powered trebuchets appeared in the Greek world and China in about the 4th century BC.

The counterweight trebuchet appeared in both Christian and Muslim lands around the Mediterranean in the twelfth century. It could fling projectiles of up to three hundred and fifty pounds (140 kg) at high speeds into enemy fortifications. source

So we built a model one - and it worked, watch the video!