Dr. Free-Ride: What do you guys want to discuss this afternoon?

Younger offspring: The human body.

Elder offspring: Yeah, how the human body works.

Dr. Free-Ride: Um, you guys know that “how the human body works” is a huge subject that we will never get through before dinner, right? You’re going to have to settle on a particular system or body part.

Younger offspring: The skeleton!

Elder offspring: The ear!

Dr. Free-Ride: Is there any room for compromise here?

Elder offspring: Well, the ear contains the smallest bone in the body.

Younger offspring: If it has a bone, I agree to the ear.

Dr. Free-Ride: Thank goodness!

Dr. Free-Ride: Tell me what you know about the ear.

Younger offspring: Well, the ear is what you use to hear things.

Elder offspring: When something makes a sound, vibrations float through the air.

Dr. Free-Ride: That’s right, sound waves.

Elder offspring: I think the ear is shaped like the way it is to catch the sounds and send them into your brain.

Dr. Free-Ride: How does the ear catch the vibrations? It doesn’t send vibrations into the brain does it?

Younger offspring: Does sound make the brain vibrate?

Elder offspring: No, the brain doesn’t vibrate. The ear sends messages into the brain about what sound it is, if it’s too loud or too soft.

Younger offspring: Or too horrible.

Dr. Free-Ride: What kind of messages does the ear send?

Elder offspring: Messages along your nerves.

Younger offspring: What is a message along the nerve like?

Elder offspring: That’s another system. I don’t really know how nerves work.

Dr. Free-Ride: That’s OK. Let’s see what we can find out about how the ear goes from the sound waves to the message to the nerve, and we won’t worry about how the nerve signal gets sent to the brain.

Elder offspring: Here, they have it in the cross-section book. The sound waves make the eardrum vibrate. Then the eardrum vibrating makes the malleus, incus, and stapes vibrate.

Younger offspring: Those are the tiny bones!

Elder offspring: Let’s see … it says that then the stapes presses on the “oval window”, and those vibrations make the fluid in the cochlea vibrate.

Dr. Free-Ride: And then?

Younger offspring: There’s more?!

Elder offspring: The vibrations moving through the cochlea make some tiny hairs wiggle, and those start the nerve sending the signal from the ear to the brain.

Dr. Free-Ride: So basically the whole ear is set up to turn sound waves into vibrations, and those into other vibrations, and it’s not until you get the special little hairs involved that the vibrations are converted into nerve impulses.

Younger offspring: And the nerve tells the brain about the sound?

Elder offspring: Then maybe the brain sends a signal back to the ear saying, “OK, I got the message!”

Dr. Free-Ride: Really? Do you think the ear needs to get word from the brain that the message was received?

Elder offspring: Actually, maybe the ear is more like a TV station that broadcasts the show even if we don’t call to tell them the show is on our TV.

Younger offspring: Some people can’t hear, though. If you’re deaf, you can’t hear the vibrations, but maybe you can feel them.

Elder offspring: If you’re deaf, maybe the hairs get stuck.

Dr. Free-Ride: Or maybe there’s a problem with some other part of the system.

Elder offspring: That’s why deaf people use sign language. But they can read and see perfectly well… unless they’re blind.