When a body forms it needs a set of instructions which it can use to assemble itself. These instructions come from two special cells. They are the sperm cells in men and egg cells in women.

The sperm and the egg cells each contain half the instructions to make a human. They must join together and share their instructions before a new human can grow. The egg and sperm join together by the sperm approaching the egg. The sperm cell is adapted for movement. Its body is simply a head and a tail. The head carries the DNA and the tail thrashes about to move the sperm towards the egg. As the egg does not move on its own it has not slimmed down like a sperm. Its DNA is contained in a nucleus which is surrounded by a large round cell body. There is food stored here to help the body begin to grow.

In the process of fertilisation a sperm head enters the egg and its DNA joins with the DNA in the nucleus of the egg. The fertilised egg then divides. It produces two cells. Each one has a nucleus with the full set of instructions for making the human body.

Cell division continues and soon a ball of cells is produced with each cell having a full set of instructions. Although each cell has all the instructions to form a body it only uses part of them. These instructions help the cell form into a particular type such as nerve cells or muscle cells. The other instructions remain in the nucleus but are not used. Each type of cell produces more of its kind as the body grows but they too only use the instructions they need and ignore the rest.

As the body grows it changes shape. Two of the first organs to grow are the brain to coordinate the body and the heart to circulate the blood. Later the limbs and sense organs develop and about nine months after fertilisation the new human body is ready to be born.

The cells continue to divide after the baby is born. As the number of cells increase the baby grows. The human body takes about eighteen years to become fully grown.