I have to create a demonstration for my physics class which will be delivered to a local elementary school. Has anyone seen anything of particular interest? The major restriction is that this has to be something the kids can interact with in some form. For example, one year a group created a block and tackle tug of war so that a single elementary student could out tug a group of high school students. I had considered building a lifter, but there's no real interaction with these. I've seen some cool stuff on university websites, but it's all either too minuscule to satisfy the project in and of itself or its beyond what I could build or have access to. Suggestions are greatly appreciated.