The first thing a flute like my shakuhachi needs to make a sound is space: the empty space inside. Whether a drum, a piano, a guitar, a bell, or even the human voice, there has to be a hollow space in any instrument for the sound to resonate around and out of. If I try to blow or tap or pluck or sing into a solid block, no sound will come out.
The second thing the flute needs to make a sound is shape. Different shapes will make different sounds. A shakuhachi will make a different sound from a French horn. So, the second thing to make a sound is the shape which gives form to the empty space and the sound it makes.
The third thing the flute needs to make a sound is breath: just the right amount of air flowing through it. So, empty space, shape and breath: take any of those three things away, and the flute won’t make a sound.