This is called a trebuchet. My kids call it a launcher, or a shooter, it's kind of like a catapault. My brother-in-law built it to launch tennis balls into the lake. It has been sitting on the sea wall at my parents house since I got here and I never thought to ask what it was. Today, Lydia, Brian, and the kids came over and hauled it out, filled the bucket with water and played "fetch" with the kids. They LOVE it. Well, Lilly, Titus, and Isabele do, Owen likes to pull the pin that launches it. The big kids jump off the end of the pier, swim out retrive the balls, swim back, climb up the ladder and run the balls back to Brian. Talk about a great way to wear them all out!