A Tube With Holes
Nearly all musical instruments are made up of two basic elements: a generator, which gets the vibration going, and a resonator, which amplifies the vibration and modifies it to create the sound of the instrument.
On the flute, the generator is the mouth hole edge against which the player’s breath is directed. When the breath meets the edge, it does not, as might be expected, divide into two separate air streams. Instead, the air stream rapidly fluctuates between going all into the hole and going all away from the hole. This sets up a rapid vibration at the head of the tube.