I call them JudoBots because of the way they seem to throw and grapple with each other during combat.
Although designed for children in grades 4-6, I think just about anyone can have fun with this. Even parent's can't resist trying it out :)
The bots are assembled from four components built separate from one another: the base, the stand, the arm, and the hydraulic system. The hydraulic system uses plastic syringes as pistons and water as the hydraulic fluid.
The cost per bot is about $4.00 and it will take up to 2 hours to complete, but you'll definitely need to schedule more time for some epic battles!
This project has been submitted for both the Teacher Contest and the Toy Challenge 2. I believe that JudoBots are a great project for both education and play in equal measure.
Prep work: 5/5
By operating a JudoBot, students will experientially comprehend the basic principles of hydraulics. Building and testing Judobots offers students the opportunity to evaluate materials based upon density, rigidity, and mass. Using the JudoBots in battle gives students hands-on knowledge of how to utilize hydraulics to effectively apply leverage to manipulate a mass. When the project is finished, students will walk away with a new appreciation of applied engineering (and an awesome new toy!)
Update Feb. 2012: After recently conducting this project with about 30 kids, I redesigned many aspects of the JudoBot. Attaching the hydraulics, which once was the most difficult step for some students, is now much easier. All of the photos and instructions are new - enjoy!