Cells

You can't see them, but they're everywhere even inside Bill Nye. This is not science fiction, it's the   science of cells.   All things that live are made from cells. You can't see them, but every part of your body, including everything   inside your body, is made from cells. Cells eat, they grow, and they make more of themselves (what   scientists call replicate).   There are millions of different types of cells. Dog cells are different from fish cells. Bird cells are different   from your cells. And inside your body, there are many different cells, each one doing a different job to   keep your body going.   Cells may be tiny, but without them, we'd be nothing.