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Why is it that when you put an empty container upside down on a surface with water, does it become suctioned ?

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2 Answers

Suction is a tricky term. In reality, there is no "suction." When you breathe in, are you sucking the air in? Right now there's about 14 lbs per sq in of pressure being exerted on your body from the mass of air above us. When you breathe, what's actually happening is that the muscles controlling your diaphragm lowers the air pressure which allows air to be forced into your lungs. 

Have you ever washed a glass then placed it upside down on a smooth counter-top only to watch it scurry across? Let's examine this. 

Suppose you rinse a glass in hot water then place it upside down on a smooth counter-top. Firstly, the excess water will collect around the glass' rim. This is important because it creates a seal. Next, the heat absorbed by the glass warms the air trapped inside. This temperature increase causes the air molecules to move faster, meaning there is a temporary increase in the pressure inside the glass. Some of the air will escape to the outside from around the rim without letting much back inside since the water acts as sort of a one way valve. This results in fewer air molecules inside the glass, and once it cools a little, results in a lower air pressure relative to that outside the glass. Pulling on the glass lowers the pressure slightly more and gives the physical sensation of being "suctioned" to the surface. Without the water seal, however, this doesn't work. 

A toilet plunger, or any other suction cup, works by the same principle. 

Here are two other aspects to this phenomenon.

The "seal" referred to above is caused by the fact that water is attracted to both the surface and the glass.  When you attempt to lift the glass, the water seal is kept in place initially.  When this happens, you are attempting to increase the volume of the air inside the glass (by pi*r^2*h, where r is the radius of the top of the glass, and h is the distance from the glass to the surface).  Increasing the volume results in a decrease in pressure.

The attraction that I mentioned is the other aspect.  Because the water is attracted to both the glass and the surface, the water acts as a kind of glue.