The main portion of all cells is filled with a fluid material. Just like a human body, the interior of the cell contains food, waste material or even extra water. On the outside of all cells is protective covering called a membrane. It is just like the shell of an oyster. Sometimes cells divide in half, forming a new cell. In other words, one cell splits in half and an exact duplicate forms. Now there are two cells. When we are growing as a baby, cell division is responsible for much of our growth.