Sound

One day, the Buddha asked his son Rāhula to strike a bell, and as the bell rang he asked his disciple Ananda, "Can you hear the bell?"

Ananda said that he could.

After the ringing had stopped and the bell grew completely silent, the Buddha asked Ananda, "Can you hear the bell?"

"I cannot," Ananda answered.

The Buddha told Ananda that his thinking was contradictory.  Ananda disagreed and said he could hear the bell when it was ringing but couldn't hear it when it was not ringing, so the Buddha had Rāhula ring the bell again, and then asked Ananda if the bell was making a sound.  .

"Yes," said Ananda.

After the bell had gone quiet again, the Buddha asked Ananda if the bell was making a sound.

"No, it's not," said Ananda.

"You just said you can't hear the bell when it's not ringing," the Buddha pointed out.  "How can you tell if it's making a sound or not if you can't hear it?"  

The Buddha was teaching Ananda that sound and hearing are not the same thing.  The faculty of hearing is present whether there's sound or not; in fact, hearing is sometimes most intense when there's no sound at all but one is waiting for something to hear.  Even though sound is not a thing but just motion, matter moving in waves ("movement and stillness" as the Buddha put it), hearing is even less substantial, being just one aspect of consciousness.  The Buddha taught that one can fully comprehended the true nature of hearing by intently listening to silence (John Cage's 4'33" comes to mind), and when the true nature of hearing was finally understood, the true nature of all the other senses would be instantly understood as well.  

And when the true nature of all the senses was finally understood, the true nature of consciousness would be understood.