We need: a hairdryer, an inflatable balloon, a tube Drop the balloon in the stream of air from the hairdryer. The balloon will start flying. The reason the balloon stays in the moving stream of air has to do with Bernoulli's Principle. Bernoulli's Principle says that pressure decreases inside a stream of flowing air. When the balloon begins to move out of this low pressure stream, the higher pressure of the air in the room pushes it back into the moving stream.