In high school my friends and I built a trebuchet. It was a fun physics project and a year or so later we decided to build a bigger and better one. We used it at a neighbor's pumpkin festival to launch pumpkins.
Years later I built a small desk sized trebuchet that I used to hurl candy pumpkins at friends in school. I also built a stage-sized trebuchet to be used on stage at a vaudeville style comedy show.