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For a long time, no one knew that plants and animals were made of. Then, in the seventeenth century, the microscope was invented. Scientists used the microscope to study plants and animals. When sections of living things were magnified under the microscope, tiny structures could be seen for the first time-structures that human eye was never able to see. In 1665, a scientist named Robert Hooke looked at pieces of cork under his microscope. He saw that the cork was made of very tiny boxes which he called cells. Other scientists believed that if cork were made of tiny cells, other living things might be made of cells, too. By using their microscopes, they found that every living plant and animal they examined was made of cells. As microscopes were improved, scientists were able to see smaller and smaller organisms. They found that no matter how large or small the organism was, it was made of cells. This led to the cell theory. According to this theory, all living things are made of cells, and all life activities of each organism take place in these cells. Today, very powerful microscopes are used to learn even more about the cell. The cell has proven to be very complex. Your body is full of cells. Every part of every living