The String Racers are rubber band powered toys that are fairly easy to build either at home or in the classroom. Materials are easy to work with and readily available, we have even been successful making our own propellors. I've had students design and build their own variations to try and improve them for distance or speed. Some other variations or challenges in design included using two propellors either side by side or at both ends, getting the racer to loop as it travels down the string or getting it to drop a marble onto a designated target. They have been a lot of fun and can be used as examples in various science principles such as potential to kinetic energy, Newtons laws of motion, measuring speed and other basic concepts.