A cell is the smallest unit in a living organism that is capable of carrying out all of the activities of life. It would stand to reason, then, that a cell could survive by itself. Indeed, there are many single-celled organisms that can be found all around us, such as an amoeba (a single-celled organism found in water); however, most of the living things we see are made of billions of cells that all work together. In the 17th century, after the invention of the microscope, Robert Hooke was able to see cells for the first time. Since then scientists have learned much about cells and have developed the Cell Theory, which says
