Ingredients: plastic hemispheres with tight fitting seal and vacuum "spout," vacuum tubing, vacuum pump

Procedure: A complete recipe follows.

1. Unite hemispheres with open vacuum spout to see that they are easily separated.

2. Unite hemispheres and use vacuum pump to evacuate sphere of air.

3. Close vacuum spout and remove vacuum pump.

4. Attempt to separate spheres.

Understanding: When the spout is open, the pressure inside the spheres pressing outward is the same as the pressure outside the spheres pressing inward. The net force on the hemispheres due to the pressure of the air within and without the sphere cancels and plays no role in the ease or difficulty of separating the hemispheres. When the sphere is evacuated, little gas remains inside the sphere. The pressure outside the spheres is still the same as before, but the pressure inside the spheres is now far weaker. We can assume the gas inside the sphere is the same composition and temperature as the gas outside the sphere, so the gas molecules collide with the same average speed inside and outside. While the same number of molecules are hitting the outside of the evacuated sphere, a far smaller number of molecules are hitting the inside of the sphere. The pressure is directly proportional to the number.

Otto von Guericke (1602-1686) is known as the inventor of the vacuum pump. In 1663 in Magdeburg (now in Germany), before the Holy Roman Emperor Ferdinand III, he created a phenomenal demonstration of the magic of a vacuum. Using teams of horses, he attempted to separate two hemispheres that had been joined and evacuated. The "Magdeburg Spheres" remained joined, held together by the great force of atmospheric pressure.