Equipment:

• 2 big cups
• 2 small cups
• 2 O-rings
• valve connected to syringe

Demo:

1.Big Cups
Each of the big cups has a built in vacuum pump and vacuum release button (pic.1). They are widely used in industry for carrying big heavy glass plates or granite/marble slabs. It is almost impossible for four students to separate the cups in the classroom (Pic. 2 and 3).

2.Small Cups
The small cups are made from clear plastic and have two concentric grooves for two O-rings. The built in air outlet with a valve is connected to the syringe (pic. 3) which the instructor can use to remove some air from the space between the rings (pic. 4). Even a very low vacuum makes it hard to separate the cups (pic.5)

Explanation: 

When we create a vacuum in between the two spheres, the atmospheric pressure exerts a force on the outside of the hemisphere. This holds the sphere tightly together.

Note:

Cups should be clean and carefully aligned. Several strokes of one vacuum pump is usually enough to hold cups tightly together.