Birdsongs, raindrops on the roof, the voices of the people you love--hearing lets you interact with the world, often from great distances. The ears are really not much more than funnels, catching sounds as they pass through the air. These sounds are then turned into nerve signals in the inner ears which lie deep inside your head.

Hearing begins when sound, in the form of pressure waves, travels through the ear canal to the eardrum--a thin, stretched piece of tissue a third of an inch across. When the sound waves make the eardrum vibrate, the eardrum in turn vibrates three bones in your middle ear, called ossicles, that are connected in a chain. These bones, the tiniest in the body, move so easily that the slightest vibration causes them to vibrate wildly, amplifying the effect of the sound waves.

The final bone in the chain sends waves through a fluid that fills a seashell-shaped organ called the cochlea. Inside the cochlea are tiny hairs, called stereocilia, that move as the waves pass over them; when the hairs move, they send electrical signals to your brain, where they are heard as sounds. Very loud noises damage the hairs in the cochlea and make your ears ring.

Q. Why do I sound different on tape?

A. When you hear yourself speak on tape, you don't hear the vibrations that your vocal cords send through the bones in your head when you talk. To see how loud these vibrations can be, try this experiment.

MATERIALS

• Two 20-inch-long pieces of string
• All-metal coat hanger

First, tie one end of each string to the bottom of the coat hanger and slide the strings as far apart as possible. Then wrap the free end of one string around each of your index fingers and gently put your fingertips in your ears. Let the hanger softly bump into something solid, such as a table. You'll feel the vibrations being conducted to your ears, as well as hear the sounds they transmit through the air.

Q. Why do my ears sometimes pop?

A. The eustachian tube connects your inner ear to the back of your throat. If air pressure builds up in the middle ear, such as when you're flying in an airplane, the eardrum cannot vibrate freely, so sounds become deadened. Swallowing, yawning and pinching your nose while blowing out through it can force air up the eustachian tube, equalizing the air pressure again, which feels like a pop.

Q. Is there a reason why I have two ears?

A. Human ears are about six inches apart, letting the sound signals from one ear reach the brain slightly ahead of those from the other. The brain uses this time difference to determine which direction the sound is coming from. Underwater, however, sound waves travel so fast that your brain can't figure out which direction they are coming from, so the noises you hear sound like they are coming from inside your head!

FACTOIDS

• Ancient Egyptians thought their ears helped them to breathe.
• Katydids and crickets have "ears" on their legs.
• The semicircular canals in your inner ear help keep your balance; they're filled with fluid and sandlike particles.
• Many earaches are actually caused by sore teeth, noses or throats.
• Snakes don't have ears. They "hear" by sensing vibrations in the ground through their lower jaws. The same goes for spiders, which sense vibrations through small hairs that cover their entire bodies.