For humans and other land animals, sound is vibrations traveling through the air. As the vibrations enter the pinna, the outer portion of the ear on the head's surface, they're funneled to the eardrum. The eardrum is a thin membrane that vibrates when it's hit by sound waves; at high frequencies it vibrates quickly, and at low frequencies, it vibrates slowly. The vibrating eardrum pushes against a group of bones, which transfer the vibrations to fluid in the inner ear. After traveling through the inner ear's fluid, sound vibrations make their way through the spiral-shaped cochlea, which is filled with hair cells. Sound waves make the hairs on the hair cells move, sending signals to the brain, which interprets the vibrations as sound.
