Light goes into your eye through an opening called the pupil. The cornea is a dome-shaped clear layer at the front of the eye that bends light, and the lens and the cornea work together to focus light on the retina. The retina is a light-sensitive layer at the very back of the eye. Light entering the eye forms an upside-down image on the retina, and the brain turns it right-side up. Each eye has a blind spot, an area of the retina with no light-detecting receptors, but an octopus does not have a blind spot.

Sound waves go into your ears and make your eardrums vibrate, and three tiny bones in the middle ear pass on the vibrations. The cochlea is a snail-shaped part of the inner ear. Hair cells in the cochlea sit on the basilar membrane and ride the wave made by sound vibrations. Low-frequency sounds are lower pitched, and high-frequency sounds are higher pitched. Hair cells near the wide end of the cochlea detect higher-pitched sounds, and the auditory nerve carries signals from the ear to the brain.

Your nose lets you smell. Special smell receptors high in the nose detect odor molecules that travel through the air, and the nose has about 10 million smell receptors. Smell receptors in the nose send messages to the brain, which identifies the smell. The brain can recognize about 10,000 different smells from the combination of smell receptors that fire. Smell also helps you taste, and flavor comes from taste and smell working together.

You use your tongue to taste, but food has to dissolve in saliva before you can taste it. Once food chemicals have dissolved, receptors on the taste buds can detect them. Taste buds can detect sweet, sour, bitter, salty and umami (savory) flavors. You may have heard that there is a map on your tongue, with sweet at the tip and bitter at the back. In fact, in humans the different types of taste cells are scattered throughout the tongue. Taste buds live for about ten days to two weeks, and then new ones grow in.

The epidermis is the top layer of your skin. Your skin has nerve endings that let you feel hot, cold and touch. Touch starts with touch receptors in your skin, which send nerve impulses to the brain. You may think every part of your skin feels touch the same. In fact, fingers and lips have more touch receptors than your back, so they feel more.

Braille is a way of reading with the fingers, using raised dots. A full braille cell has six raised dots arranged in two rows of three, and Louis Braille was born in Coupvray, France, on January 4, 1809. American Sign Language is the primary language of many North Americans who are deaf or hard of hearing, and it is expressed by movements of the hands and face. You may have heard that it is English on the hands. In fact, ASL is a language completely separate and distinct from English. Fingerspelling is part of ASL and is used to spell out English words, and there is no universal sign language.

You may have heard that you have only five senses. In fact, you also have a sense of balance. The semicircular canals in your inner ear tell your brain when your head turns, which helps you stay balanced. When your head moves, fluid in the semicircular canals moves tiny hair cells, which send a signal to your brain. You may think your eyes alone make the picture you see. In fact, the optic nerve carries signals from the eye to the brain, and all your senses send messages to your brain.

Words to know

• cornea: clear front layer
• frequency: pitch measure
• braille: raised dots
• fingerspelling: spelling with fingers
• saliva: spit