18 A2+ bones

CHAPTER 1 Bones WHAT DO YOU KNOW ABOUT BONES? When you see bones in a museum, they look hard, dry, and dead. But when they are inside our bodies, they are living things. They grow and change, and they can even repair themselves when they are broken. Bones are amazing things! When a human baby is born, it has about 270 soft bones. When the baby starts to grow, those bones get bigger and harder, and some of them join together. Adults have only 206 bones. By the age of about 25, a person's bones stop growing. Now the person is as tall as he'll ever be!

One of the most important parts of the skeleton is the skull. The 22 skull bones are like a case for the brain. The smallest bone in the body is found here. It is called the stapes, and it is part of the ear. It is only 2.5—3.3 millimeters long. The only skull bone people can move is the jaw. It is used to open, close, and move the mouth. A person's spine goes down the middle of the back. It is very important because people need it to stand up straight. It also helps people move and bend. The spine isn't one bone, but 26 small bones that look like rings. The ribs are connected to the spine. Most people have 24 ribs, 12 on the left and 12 on the right. Some people, however, are born with one or two ribs more or less. The ribs are like a case for the heart. So if someone falls, the ribs might get hurt, but the heart will be safe.

At the top of the leg is the femur. This is the longest, biggest, and strongest bone in the body. When people stand, walk, or run, all their weight falls on this bone. More than 50 percent of a person's bones are in the hands and feet. There are 27 in each hand and 26 in each foot. This means we can move our hands and feet in many different ways. Bones are very strong and very light. If a person weighs 100 kilograms, only about 15 kilograms of this is bone.

But what happens to bones when people die? Well, sometimes bones are not destroyed for a long time, maybe for millions of years. Special scientists called paleontologists study old bones and teeth. They ask questions like these: Have all the skull bones joined together? (This tells them the person's age.) What are the teeth like? (This tells them about what the person ate.) How long is the femur? (This bone is about a quarter of a person's height, so they can guess how tall the person was.) Bones can be the key to the secrets of history!

CHAPTER 2 Dinosaur Bones WERE DINOSAURS BIG OR SMALL, FAST OR SLOW? DID THEY LIVE ALONE OR IN FAMILIES? Today, dinosaurs are everywhere: but only in books, movies, and museums. Real dinosaurs lived in our world between 65 and 230 million years ago. That's about 62 million years before the earliest humans. So how do we know so much about them? When an animal dies, the soft parts of its body are destroyed by nature. But sometimes the hard parts, like the skeleton and the teeth, are not destroyed. This happens when the animal's bones are covered quickly by rock. If the bones stay under this rock for a long time, they become fossils. And scientists can study fossils millions of years later!

The first dinosaur fossils were probably discovered thousands of years ago, but people didn't know what they were. The Chinese thought they were dragon bones, and they used them in medicine. And in England in 1676, an Oxford University teacher found a very big leg bone, but he believed it belonged to a giant. In 1841, the British scientist Richard Owen gave the name "dinosaur" to a group of animals. From his studies of bones, he showed that they all had the same kind of pelvis and straight legs (a bit like the human pelvis and legs today). He took the name from the Greek language: deinos meaning "terrible" and sauros meaning "lizard."