The Magdeburg Hemispheres is a classic physics experiment that demonstrates the incredible pressure the atmosphere around us exerts on our bodies and everything else.
The apparatus of the experiment consist of two brass hemispheres that fit together to form an air-tight seal. One hemisphere has a tube that can be attached to a vacuum pump and a stop cock to seal it off. When the air is sucked out from inside the hemispheres, and the valve is closed, the two halves are held firmly together by the air pressure of the surrounding atmosphere. It is nearly impossible to pull the hemispheres apart by any number of hands. Once the air is let back in, the halves fall apart easily. This simple demonstration of the pressure of the atmosphere was first made by German scientist Otto von Guericke in 1654. When the rims were sealed with grease and the air was pumped out, the sphere could not be pulled apart by teams of horses.
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Source: amusingplanet.com