4M Tin Can Edge Detector

First, there's the learning experience of building it (you provide the empty soda can and small screwdriver). Then comes the fun and suspense of watching it move across a table, get right to the edge, and whoa! How does it know?! Requires 1 AAA battery. For ages 8-12 yrs, from Toysmith.
Kit includes:

• Base with wires
• Base cover
• Lower can ring
• Upper can ring
• Gear top plate
• Battery cover
• 2 lower arms
• 2 upper arms
• 2 transparent covers
• Motor with wires
• 2 axles
• 2 eye supports
• 2 eyes
• 2 sets of bolts
• 2 gear wheels
• 2 terminal caps
• 5 small screws
• Instructions