Coanda effect Experiment

When you blow into the square jug, the air has to change direction and it will follow the surface until it hits a sharp corner, at which point the flow will tend to separate from the surface of the square jug and carry on off to the sides as seen in the video.

On the other hand you blow at the circle bottle, the air will tend to stick to the surface even as the surface curves, this is called the Coanda effect. This way the air curves all the way around the circle bottle, and can blow out the candle.

The air is deflected from its original path far less by the bottle than the square jug which is the reason that cars tend to have very smooth streamlined shapes, which use far less energy to move fast.