Key concepts
The five senses
Sound waves

In the popular pool game "Marco Polo," one sightless swimmer calls out "Marco" and seeks out other swimmers when they respond "Polo." If you've tried it, you'll know it's not an easy game. Every splash, whisper or giggle is a clue, but these hints can be surprisingly misleading. In this activity you'll learn a little bit more about your sense of hearing and how it works.

Most of us chiefly rely on eyesight, which is part of why we become so disoriented when forced to resort to a different sense. Nevertheless, our ears share a lot of important information about the world around us. From the shout, "Fire!" to a relaxing melody, your ears convey useful signals and you wouldn't want them to deceive you.

Sound travels through the air in waves. Your ears are specially equipped to receive and understand these waves. Each ear collects and channels sound waves, transforming them into vibrations. Within your inner ear tiny hair cells respond to these vibrations and send signals that your brain can decode and interpret as a variety of sounds. But why exactly do we have two ears instead of just one? Try this activity and find out.

•    Adhesive tape (or chalk if outside on pavement or sidewalk)
•    Meter stick or measuring tape
•    Blindfold
•    Earplugs (optional)
•    One or two noisemakers: a clock that audibly ticks, music box, small musical instrument, cell phone, pennies in a closed jar, a book (optional)
•    Pencil and paper

•    Blindfold your partner and ask him or her to stand on the central X. Record or draw the direction your partner is facing and ask your partner to remain in that spot throughout the activity.
•    Explain to your partner that you will stand in different parts of the room and say his or her name. Your partner will then guess where you are (left, right, in front, behind) and how far away you are standing.
•    When you're both ready to begin, ask your partner to cover one ear using a hand or inserting an earplug.
•    Stand on one of the lines you have made in the room. Say your partner's name in a normal speaking voice. Ask him or her to guess where and how far away you are standing. Did your partner guess where you were standing?
•    On a piece of paper, record which ear your partner used, where you actually stood (distance and direction from the central X), and what your partner guessed.
•    Move to a different location and repeat. Try this from different lines all over the room and record your partner's guesses. Is your partner better at guessing when you're close by or far away? How well does your partner guess distance? What about direction?
•    Once you've tried several spots in the room, ask your partner to use both ears and try the above steps again. Is your partner better at guessing distance with two ears than one? What about direction? You can also try the above steps after asking your partner to cover the other ear. Is one ear better for guessing locations and distances than the other? Does it make any difference which ear is covered?
•    Extra: Try this activity and vary how loudly you say your partner's name. Does volume make it easier or more difficult for your partner to judge distance? What about direction? Is a whisper harder to locate than a shout?
•    Extra: Instead of speaking, try using an object that makes noise such as a ticking clock, music box or cell phone—or use a homemade noisemaker by shaking pennies in a closed jar or opening and sharply closing a hardbound book. How does a noisemaker compare with a voice? Are some sounds harder to locate than others? Try this activity while your partner stays perfectly still, then allow your partner to move his or her head. Does head turning or tilting make a difference?
•    Change places with your partner and compare your findings!

Observations and results
Were two ears better than one in identifying distance and direction? Was direction more difficult to guess with one ear when you and your partner were closer to one another?

Each ear receives information that is sent to your brain. Because your ears are not side by side, they receive different information. If someone standing to your left claps his hands, your left ear will receive this sound wave more quickly than your right one. In addition, the clap will sound louder in your left ear than in your right . Your brain uses these differences to better understand where a sound is coming from. This can also explain why—as you may have noticed—it's hard to tell the difference between a sound directly in front of or behind you, even if you are using both ears. When the sound source is exactly equidistant to both ears, they receive very similar information and your brain has fewer clues as to where the source may be.

If indoors, remove tape from the floor.

More to explore
One Ear Is Not Like the Other, Study Finds from Scientific American
Our Sense of Hearing from Neuroscience for Kids
A Brain Map of Auditory Space from the Howard Hughes Medical Institute
Your Ears from KidsHealth
How Hearing Works from How Stuff Works