I've been thinking. You know that Zen Koan, "If a tree falls in a forest and no-one is around, does it make a sound?"
Previously I've always said yes, since the sound is caused by the energy being transferred through the particles in the air, which is caused by the tree falling.
But really, what is the sound? Is the sound the energy, the transferal of said energy, or is it the interpretation of the vibrations of the air particles?
If the latter is the case then the question should not just be whether it makes a sound when no-one is around, but whether it ever makes a sound.