The Cell

Let's talk about the cell. The cell is the basic unit out of which all known life is built. It's a simple thing: a water (it's called cytoplasm, but it's 90% water)-filled membrane with a few internal parts that perform simple functions. 

onion cells

A cell is pretty simple, but if you get enough of them interacting the result can be very complicated and interesting  For example, your typical human is made of about 100 trillion cells. Each is about ten microns (0.001 mm) across. That's pretty small, but still pretty easily observed with a microscope (try it with a bit of skin or blood). Not everything in the body is made of cells, but everything you would consider living is. The enamel coating your teeth, or the keratin that makes up your fingernails are not cellular.

We are, obviously, made of many cells working together, but many living things get by as just a single cell. Bacteria, amoebas, and algae are all single-celled. The largest known single-celled organism is the xenophyophore. That's a mouthful but sorry, there isn't a laymen's name. That's because this thing is completely removed from the human experience. It's only found at the bottom of the earth's deepest oceans. In these depths, they are apparently abundant, but it's so difficult to get down there, and so much more difficult to retrieve something intact, that we know very little about them. What we do know is that the suckers can be almost eight inches across. 

That's a really big cell.