 Every living organism is made up of cells, from plants to the lowliest worm to the most complex human being. But what is a cell? Well, a cell is a small self-contained unit within a whole organism, kind of like a room within a house. It is the smallest unit that can be classified as a living thing. We can think of them as the building blocks of life. So how many cells do you think you have in your body? A thousand? A million? A million million? More. We contain around 50 trillion cells. Have a look at your little finger. There's probably about 10 billion cells in there. Now cells come in all different shapes and sizes. Occasionally cells can be large enough to be seen by the human eye. For example, the yolk of a chicken egg is actually a single cell. And some cells are really long, such as a nerve cell that runs the entire length of your spine. However, most cells are really, really tiny and can only be seen under a microscope. They are so tiny that you could fit about 10 of them across one of your hairs. Now there are many different types of cells, and each cell carries out its own jobs. Some of these jobs are common to all cells, such as making proteins and producing energy, while some jobs are specific to the type of cell. A human sperm cell has to be able to swim, while the cells that produce the peacock's tail must produce beautiful coloured pigments. Sometimes cells can exist on their own, such as a simple bacterium or a single-celled organism called an amoeba. But mostly cells work together to form more complex multicellular organisms, such as animals, plants and insects. In a multicellular organism, as well as doing its own jobs, each cell also works together with the other cells around it to carry out an overall job. For example, the cells of the heart work together to pump blood around your body. So cells come in all shapes and forms and have all sorts of jobs, but they are all fundamental building blocks of living organisms. Watch this video to learn more about different types of cells.