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Why the pressure of atmosphere doesn't crush you when you e.g. walk outside? I mean the density of air is $1.26 kg/m^3$, so with $100 km$ above us, it exerts much pressure on you when you walk outside.

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If you were a completely empty shell you'd likely be crushed immediately on finding yourself in the earth's atmosphere. But you are filled with stuff (blood, flesh, bones) which is also at approximately atmospheric pressure. If you consider a point on your skin, the pressure of the air on the outside pushing it in is exactly matched by the pressure of the contents of your body pushing it out. So the net force is zero.

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    $\begingroup$ Just to add an experiment to your explaination, try the following experiment: get a can of soda and put a little water on it (like 0.5 cm above the base) and a bowl with cold water. Now, put the can on the kitchen and wait until the little ammount of water you putted on the can starts to evaporate. With some heat-gloves, take the can off the kitchen, turn it over (with the hole where you drink from the can looking down) and put it inside the cold water. The atmospheric pressure will do the rest! $\endgroup$ – Néstor Apr 16 '12 at 21:11
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    $\begingroup$ Here's what happens when you have an object that doesn't contain material at a high enough pressure to oppose atmospheric pressure: youtube.com/watch?v=Zz95_VvTxZM $\endgroup$ – Dan Piponi Apr 17 '12 at 22:32
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To add to Dan's answer - the same thing also applies indoors.

Even with a roof over your head the outside air presses on the windows, doors and walls with the same air pressure, and so compresses the air inside the room to the same pressure.

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