If we put 2 directories together, then hold them by their spines, it is really hard to pull them apart. This is because the air pressure on the pagers makes the two directories stick together well.
It is easy to knock a ruler off a table. Just a little tap will do, unless you spread newspaper on top. The air pressure pushing down on the paper makes it hard to knock the ruler. 
The bigger the piece of paper you have, the harder it is to knock the ruler.
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