When a student blowing above a piece of paper, the paper will rises upward. Another student blowing with a straw between 2 ping-pong balls suspended close to each other on strings. As a result of the blow, the two balls move closer to each other.
These are due to the difference in air pressure. A fast flowing stream of air results in a lower pressure. This can be explain by Bernoulli’s principle.
Bernoulli’s principle states that when the velocity of a fluid is high, the pressure is low, and when the velocity is low, the pressure is high.