Observations and results
When the ball is suspended in the airstream, the air flowing upward hits the bottom of the ball and slows down, generating a region of higher pressure. The high-pressure region of air under the ball holds the ball up against the pull of gravity.
When you pull the ball partially out of the airstream, the air flows around the curve of the ball that is nearest the center of the airstream. Air rushes in an arc around the top of the ball and then continues outward above the ball.
This outward-flowing air exerts an inward force on the ball, just like the downward flow of air beneath a helicopter exerts an upward force on the blades of the helicopter. This explanation is based on Newton's law of action and reaction.
When you approach a wall with the balanced ball, the high-pressure region under the ball becomes a region of even higher pressure. The air that hits the bottom of the ball can no longer expand outward in the direction of the wall. The higher pressure drives the ball to a greater height.
More to explore
Lift, from Aviation for Kids
How Do Helicopters Fly?, from Decode Science
Wind Tubes (pdf), from Exploratorium
Bernoulli Levitator, from Exploratorium
Hair Dryer Gravity Defier was developed by Exploratorium, and is on page 62 of Exploralab. Created by Exploratorium, Exploralab: 150 Ways to Investigate the Amazing Science All Around You is a book that takes curious kid scientists, ages eight–12, through 24 hours' worth of household investigations, experiments and discoveries.