Introduce the concept of center of mass with this inquiry based lesson. First, lay the center of mass bottle on its side. Your students will think this is simple enough. Then, pick up the bottle and secretly allow the counterweight inside the bottle to fall into your hand. Hand the empty bottle to a student and ask them to lay it on its side. After several failed attempts the bottle will remain upright. Pick up the bottle, secretly insert the counterweight again and again show students how easy it is to lay the bottle flat. The effect is achieved by shifting the center of mass of the bottle. When the counterweight is inserted, the bottle will lay on its side because its center of mass is near the middle of the bottle. Remove the counterweight and the center of mass shifts to the bottom of the bottle and it will right itself when placed on its side. Two bottles and two counterweights are provided. Bottles are 1 1/2" tall.