Cells are basic structures that make up all living things. The smallest organisms, like bacteria, can be made up of only a single cell; these are known as single-celled organisms. Other organisms, like animals or humans, are made up of many, many cells. The size of a typical cell is very small, usually on the scale of 10⁻⁴ or 10⁻⁵ meters. This is small enough that we can’t see it with our eyes, but only under a microscope where cells are readily visible.