An elephant, a sunflower and an amoeba are very different on the outside. On the inside, they are all made of the same building blocks. Each and every living being on Earth is made of cells. The smallest living things have only a single cell. The human body has trillions of cells. Cells are living things' simplest building blocks. Each cell is made by other cells. This knowledge is well understood today. It is part of a science field called cell theory. Scientists did not always know about cells, though. Tiny Holes We Now Know As "Cells" The discovery of cells would not have been possible without advancements in microscopes. Scientist Robert Hooke improved how microscopes worked in 1665. He made what is called a compound microscope. It used three lenses and light. It lit up and enlarged whatever you put under it. Hooke placed a piece of cork under the new microscope. It allowed him to see something amazing. To him, the cork looked as if it were made of tiny holes. He called them "cells." That's because they reminded him of the cells in a monastery, where monks live. The Microscope Shows Tiny Living Things Soon after Hooke's discovery, Dutch scientist Antonie van Leeuwenhoek found other hidden, tiny living things. They are called bacteria and protozoa. Van Leeuwenhoek perfected the simple microscope. It only had a single lens. It could zoom in around 200 to 300 times the normal size. What van Leeuwenhoek saw with these microscopes was bacteria and protozoa. He called these tiny creatures "animalcules." Van Leeuwenhoek became very interested in these creatures. He even took a look at the plaque between his teeth under the microscope. He saw "little living animalcules" on his teeth, he wrote in a letter. Plants And Animals Are Made Up Of Cells In the 1800s, scientists began taking a closer look at animals and plants. Scientists could easily tell that plants were completely made up of cells. That's because plant cells have a layer on the outside called a cell wall. This was not so obvious for animal cells, though, since they do not have a cell wall. German scientists Theodore Schwann and Mattias Schleiden studied cells. Schwann studied animal cells and Schleiden studied plant cells. These scientists saw key differences between the two cell types. They had the idea that cells were the simplest units of both plants and animals. Why We Look Like Our Parents A scientist named Rudolf Virchow made another important discovery, in 1855. He found that all new cells are made by existing cells. They copy themselves. Later, scientists began to focus on 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 are responsible for passing down genes from parents to children. This is why children look like their parents. Scientists Are Looking At Stem Cells New discoveries were made 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 problems such as heart disease. The discovery of the cell has been far more important for science than Hooke could have ever dreamed in 1665. It gave us an understanding of the building blocks of all living things. Today, scientists are working on ways for each of us to grow stem cells from our very own cells. We could use them to understand how diseases work. All of this and more grew from simply looking at a cork under a microscope.