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Based on a design by British scientist Neil Downie, this is an exemplary model. It demonstrates the complex force of flexing muscle. Construct an arm that contracts and extends. Consider Leonardo daVinci's study of anatomy and the robot mechanics it inspired. Domo arigato, Robot Armo!
Parts 
Step 1
Glue a short dowel into the hole in the base, and glue the peg and frame together. Make sure the half-arc is facing outwards. 
Step 2
Slide your small wheel into the frame and push the long, smooth dowel through. Make sure this wheel spins freely! You may have to sand down the wheel or the axle. 
Step 3
Glue the handle into the crank wheel and push the long, spiral-textured crank axle through the frame. 
Step 4
Slide the crank wheel and the cap onto the axle with a rubber o-ring at either end to hold in place. Your rubber band will stretch across the crank wheel to the smaller wheel above. 
Step 5
Push the other spiral-textured dowel through the frame sideways. The Robot Arm will mount here. 
Step 6
Screw the screw-eyes into the small drilled holes on the arm "bones." Use two short dowels to pin the joints, and use o-rings to hold them in place. 
Step 7
Push the loop end of the "tendon" string through the screw-eye at the wrist. Put the other end through the loop and pull tight, lanyard-style. Thread the end through the screw eye at the elbow. 
Step 8
Put the end of the tendon string through your bead and slide this over the top axle, pinning the string in place. 
Step Done
Turn the crank and watch as the contracting tendon moves your mechanical arm. He's waving! 