Think about all the things you do with your eyes. You watch TV and read books. You surf the Internet. You keep your eye on the ball when you play sports. You see your family and friends. Your eyes are your windows on the world.
Your eyes are like cameras that focus on what is in front of you. Your eyes work together with your brain to create a picture of the world. The process of creating the picture starts when light rays enter your eyeball.
LOOKING AT YOUR EYEBALL
Look at one of your eyes in a mirror. Your eyeball is round. The inside of your eye is a transparent (see-through), jellylike material called the vitreous humor.
What is your favorite food? Some people love pizza. Some people love ice cream. Not many would say broccoli is their favorite food.
But you can’t eat only pizza and ice cream all the time. You also need fruits, vegetables, and other kinds of foods. Water is also very important for your body.
WHY DO WE NEED FOOD?
You need food for many reasons. You need food in order to live. You need food to satisfy hunger. Your stomach feels hungry when you need to eat more food.
Hair is one of the things unique to the mammal world. Dogs, cats, lions, squirrels, and seals—they all have hair. So do humans. Hair grows only on mammals.
WHAT IS HAIR?
Hair is an extension of your skin. It’s made up of old skin cells that are filled with a protein called keratin. Pack a lot of these cells in a long, narrow line and you have a hair. Keratin is used to make fingernails, claws, scales, beaks, feathers, and other skin attachments, too.
Have you ever heard a news reporter talk about DNA? Reporters talk about DNA found at the scene of a crime. They talk about police finding DNA “fingerprints.” Police sometimes use DNA as a clue to find out who committed the crime.
WHAT IS DNA?
DNA is a substance that makes up genes. Everything alive has genes. Plants have genes. Animals have genes. You have genes.
Did you ever send a valentine with the shape of a heart on it? Did you ever hear someone say, “That came straight from my heart?” People talk about hearts a lot. People have always known that hearts are very important.
You have a heart. Your heart does not look like a valentine heart. Your heart is a pump. When you run very fast, your heart pumps hard and fast. You can feel your heart pumping, or beating.
WHAT DOES A HEART LOOK LIKE?
Your heart looks like an upside-down pear. It is about the size of your closed fist. It is almost in the middle of your chest. It is just off to the left side.