A cell is the basic unit of life - it is like a very tiny room. In every living thing there are cells, which are like the building blocks of our bodies. Cells can be very small, even smaller than the eye can see. All cells have a membrane that acts like a wall and keeps things inside them. Inside the cell there are many structures that each have their own job. For example, the nucleus is like the "brain" of the cell - it helps the cell do its job. Cells also contain little packets of energy called mitochondria that help to power the cells. Cells make up every living thing, from single-celled organisms like germs, to multi-celled organisms like humans.