An elephant and a flower are very different on the outside. On the inside, though, they are both made of cells. Each and every living being on Earth is made of cells. The smallest living things have only a single cell. Our bodies have trillions of cells. Cells carry out specific jobs in the body. Some cells become your skin. Others become your organs inside you. This is well understood today. It is part of a type of science called cell theory. Scientists did not always know about cells, though. Under The Microscope The cell would not have been found without the microscope. Scientist Robert Hooke made a new kind of microscope in 1665. He made what is called a compound microscope. It used three lenses and light. It lit up and zoomed in on whatever you put under it. Hooke placed a cork under the new microscope. It allowed him to see something amazing. To him, the cork looked as if it was made of tiny holes. He called them "cells." Then, Dutch scientist Antonie van Leeuwenhoek made a new microscope. It only had a single lens. It could zoom in around two hundred to three hundred times normal size. It allowed van Leeuwenhoek to see tiny creatures. These were bacteria and protozoa. The Building Blocks Of Plants And Animals In the 1800s, scientists began looking closer at animals and plants. German scientist Theodore Schwann studied animal cells. Another German scientist, Mattias Schleiden, studied plant cells. They found that cells were the simplest building block of both plants and animals. A scientist named Rudolf Virchow learned something important in 1855. He found that all new cells are made by existing cells. They copy themselves. Genes Are Why We Look Like Our Parents Later, scientists began to look at genes. Genes tell the body how to grow and work. Chromosomes are like threads inside of cells. They carry a series of genes. In the 1880s, Walter Sutton and Theodor Boveri discovered what chromosomes are for. They pass down genes from parents to children. This is why children look like their parents. Stem Cells Have Special Jobs In the 1900s, scientists learned about stem cells. Stem cells are simple cells. They still have to develop into cells with more specific jobs. This means they can grow into many different parts of the body. They could become part of your skin or your heart, for example. Stem cells are now used to treat health problems. They help people with heart disease, for example. Cells In The Future Today, scientists are working on new ideas. They want each of us to be able to grow stem cells from our very own cells. We could use them to understand how diseases work. All of this started by simply looking at a cork under a microscope.