Here is an excellent instructional video from sciencetoymaker.com on making a simple and inexpensive Cartesian diver experiment from a straw and some paper clips. The video is not only clear and step by step, but also explains the science behind it as well as trouble shooting and how to handle a big class project. This is a great science craft project or summer school activity. There are other variations. I saw where you can color and cut out a sea creature from a plastic file and tape to the straws. The key to the diver is that it needs to be weighted and just barely float. I also saw diver from a plastic pipette. Put a little plastic skirt on it and make a squidy diver. There are also divers that spin.