Sci Fri: Catapults
Super fun engineering experiment that our six year old could pull off himself, courtesy of Steam Powered Family. https://www.steampoweredfamily.com/catapult-cannon-stem-activity/
All you need is nesting paper towel rolls (we used one aluminum foil roll and a paper towel roll), rubber bands, tape and a stick (chopstick or steel straw in our case).
This wasn't necessarily the design for Greek catapults, but it was a fun and simple activity to get an idea of how catapults function.