Hero of Alexandria (c. 62 A.D) designed a water fountain that used compressed air to lift water to a point higher than its origin. Using your own soda bottles and this device, you can demonstrate the same principle. Water leaving the upper bottle creates a partial vacuum and enters the lower bottle compressing the air there. To balance out the pressure, air flows from the lower bottle to the upper bottle. The result is a spurting bubbling fountain that forms every time you turn this demonstration over. Additional experiments are suggested.