The hair dryer produces a fast-moving column of air rising straight up. Fast-moving air, as Daniel Bernoulli discovered in the 18th century, has lower pressure than still air. Think of it like a busy motorway: cars moving at high speed keep to their lane and barely push sideways. 

The still air surrounding the motorway pushes in from the sides with full force. The balloon, sitting above the hair dryer's airstream, finds itself in exactly this situation: low pressure below from the fast air, normal pressure pressing in from all sides. That inward pressure from the surrounding air is what holds the balloon up — gravity is being outgunned by a pressure difference.

The coin inside the balloon is the secret ingredient. Without it, a balloon is light and round with no preferred orientation — it spins, wobbles, and drifts randomly. The coin sits at the bottom by gravity, giving the balloon a heavy end and a light end. The heavy end stays down. The balloon stops tumbling and sits steadily in the airstream, like a ship with ballast in the hull.

The tilting behaviour reveals the Coanda Effect at work. When you tilt the hair dryer sideways, the airstream tilts with it. Instead of falling away from the angled airstream, the balloon follows it — because moving air clings to curved surfaces and bends to follow their shape. 

The curved surface of the balloon deflects the airstream sideways, which lowers pressure on the balloon's near side and higher pressure from the far side keeps pushing it in. The balloon chases the airstream the way a satellite hugs its orbit — always correcting, never escaping.

The same two principles — Bernoulli and Coanda — appear together in every aircraft wing, every helicopter rotor, and every industrial hovercraft ever built. Your hair dryer just demonstrated the physics of flight using a balloon and a coin.

To see the same self-correcting Coanda behaviour on a smaller scale — and without a hair dryer — try [The Invisible Cushion], where a handmade card cone and a single breath do the same job. And if you want to see Bernoulli working sideways instead of vertically, [The Stubborn Balloons] shows what happens when fast air runs between two objects instead of under one.