Every audience member has a pen and a table tennis ball. It is possible to use the lid of the pen and make a table tennis ball float, if you blow hard enough.
How does it work?
When the airflow hits the bottom of the ball, it creates pressure which pushes the ball upwards. To explain why the ball does not fall down to the side, you need to understand the Bernoulli principle: a fast flowing current leads to low pressure. That is, where the air flows quickly around the table tennis ball, the air pressure is low. Move the ball from the middle of the stream to the right a little bit, for example, and the flow on the left side become faster. This faster flow leads to less pressure, and the ball is pushed back from the high pressure on the right to the lower pressure on the left.
