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from Spangler Science
Fluids, including air, have friction similar to solid objects since they are all made of molecules. This plastic tube helps demonstrate this. Unroll it, tie a knot in one end, and try to inflate it by blowing into the open end. It isn’t very effective up close, but if you hold the opening about 1 foot away, just one full breath can inflate the whole bag. This is because the blown air pulls along surrounding air by means of viscous friction called entrainment. As a result, the volume of air entering the bag is larger than the volume of air leaving your mouth.
This phenomenon is often credited to Bernoulli’s principle while others argue that Bernoulli’s principle is exclusive from entrainment. Research it yourself. What do you think?