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 individual is developed from cells. No two individuals are alike. Most cells are so tiny; they can be seen only under a microscope. Some cells are so small, you could put thousands of them on the period at the end of this sentence. Different kinds of cells have different shapes. Some cells are round. Some are oval. Some are shaped like spiral rods. And some are long and skinny. Certain kinds of cells group together to form a tissue. A tissue is a group of the same kind of cells that work together to do a certain job. Tissues group together to form an organ. An organ is made of groups of the same kind of tissues that work together to do a certain job. The different kinds of cells each do a special job.