What happens

This is a simple demonstration dealing with Bernoulli’s Principle. The demonstrator will hold a piece of paper close to their lips, and blow across the top of the paper as it hangs down from their hands in front of their lips. The paper will respond by moving up, toward the stream of moving air. It would seem the paper would move away from the stream of air, but it does not. It moves toward the stream of air. 

How it happens

Bernoulli’s principle states that fluids in an area moving faster than the the surrounding area possess less pressure. Faster-moving fluid, lower pressure. (In general, fluids include liquids and gasses. Air is a gas and as such is classified as a fluid.) When the demonstrator holds the paper in front of his mouth and blows across the top, he is creating an area of faster-moving air. The slower-moving air under the paper now has higher pressure, thus pushing the paper up, towards the area of lower pressure. Perhaps you have noticed, for example, that an empty mayonnaise jar in your sink with water in it will move in, directly under the water faucet as you turn on the water. This is the same thing as we observe here. The water that is moving possesses less pressure, and the jar is pushed toward that area by the water that is not moving which has greater pressure.

Vocabulary

air pressure: Pressure exerted by the weight of air on an object in that air. 

atmospheric pressure: This is also the pressure caused by air, but usually thought of as the normal value or standard value of 14.7 pounds per square inch. The atmospheric pressure at the place where we are on the face of the earth is usually less than this because we are above sea level, and hence have less "weight" pushing down on us. 

high pressure: A space where the pressure is more pounds per square inch than a neighboring space. In this demonstration there is a high pressure zone below the toilet paper roll, which causes the paper to lift.

low pressure: A space where the pressure is fewer pounds per square inch than a neighboring space. In this demonstration, a low pressure zone is created by the moving air across the top of the paper.

molecular motion: The name given to the intrinsic property of all matter that is at normal temperature where the molecules of that matter are in a constant state of motion. We will be using this concept to help describe what causes air pressure. 

pressure: In the context of our show, it will be the number of pounds per square inch exerted by gaseous molecules. When the number of pounds per square inch are multiplied by the number of square inches on an object, it is possible to determine the total force, the push on the object.