Here is a mesmerizing marble machine that uses solar power to keep you entertained for a while. The Solar Marble Machine has a solar cell with a smart circuit to keep droping the marble down the spiral. In sunlight, the motor pulse every few seconds and drop a ball every minute. You will have to wait a bit longer indoors. Here is a neat part: the good folks at Solarbotics have a kit for you with all the parts needed to build a Marble Machine. Not too expensive either.