Explain why

To keep a piece of paper horizontal, you should blow over, not under, it

When we blow over a paper velocity of air above the plane of the paper increases. As a result Kinetic energy above the plane of paper increases.

According to Bernoulli's principle

As K.E increases, Pressure energy above the plane of paper decreases below the atmospheric pressure. The pressure below the paper is atmospheric pressure As a result pressure above the paper is less than pressure below the paper. So the paper remains horizontal and does not fall.