I originally made this project for the MAKE blog. It's a fun experiment that led to a lot of discussions about perpetual motion and free energy. My version does not exhibit either of these properties, but you may be able to fool people into thinking it does.
This is a really easy build and would be a perfect project for to build with your kids. Maybe you could even sneak in a lesson on fluid dynamics or perpetual motion?
Step 1: What You need: Supplies
(3) 16.9 FL OZ Water bottles
(1) 9" length of tubing
(1) 11" length of tubing
(1) 15" length of tubing
Small amount of clay
Note: The tubing is for aquariums and is 3/16" thin wall rigid tubing. Almost any tubing would work, even flexible, but the rigid makes it really easy. I was able to pick some up at a local pet supply store for about $0.50 per foot.