Jim and Harry's Trebuchet was built in part at the Exploratorium and was tested in the Marin Headlands. They were not allowed to chuck rocks into the ocean so

Jim and Harry's Trebuchet was built in part at the Exploratorium and was tested in the Marin Headlands. They were not allowed to chuck rocks into the ocean so they marked all of the rocks used for their tests with the tag "This is not a rock", a paean to surrealist Rene Magritte.