We know about cells through making observations on them with a microscope. There are many different types of cell. Some are flat like pavement flags. Others are cubes or like rectangular blocks. The surface of most cells are smooth but some such as those lining your windpipe have one surface covered in tiny hairs that wave to and fro. They help move dirt out of your lungs.

There are three main parts to cells. They are the outer skin or cell membrane, the jelly - like contents and a dark speck called the nucleus.

A cell is a living thing. It needs food and oxygen to keep it alive. The cell gets its food and oxygen from the blood. The food is dissolved in the blood and the oxygen is carried from the lungs by the red blood cells.

The food and oxygen pass from the blood through the cell membrane. Inside the cell the oxygen is used to release energy from the food. The energy is used by the cell to keep it alive and to help it grow.

When energy is released from food carbon dioxide is made. Carbon dioxide is harmful to the cell so it is removed. The carbon dioxide passes through the cell membrane and into the blood. From here it travels to the lungs and is released from the body when a person breathes out.

All the activities of the cell are controlled by the nucleus. If most cells lose their nucleus they quickly die. There is one exception to this rule. A red blood cell starts life with a nucleus but loses it as it fills up with a red substance to help it carry oxygen. This red substance is called haemoglobin.

Further studies on cells have shown that their shapes may help them perform a particular task which keeps the body alive. Skin cells are thin sheets which overlap each other and form a covering over the body which keeps out harmful germs. Most nerve cells are long threads. They send messages between different parts of the body. The messages are in the form of tiny electrical currents. In the digestive system some box shaped cells make juices which they release onto food to break it down.

The billions of cells are organised in an orderly way in the body. Cells of the same type form groups called tissues like our skin.