3. Moving Boat with a Rudder - One knob is used to adjust the rudder (for direction) and the other is rotated to compress a spring to ultimately rotate the fan underneath the boat. The student can thus understand concepts regarding force in water as well as resistance caused by the rudder that ultimately steers the toy. The construction cost is approximately $4-5 because there are only a few moving parts and they're made of plastic. By thinking of this toy, I learned that kids often learn better by experimenting and then rationalizing their observations with theory, instead of doing it the other way around.