A lot of people wish we knew the whole answer to your question, but no one does. Here is some of what we do know.

Your body is made up of trillions of cells (a trillion is 1,000,000,000,000). These cells are so small that you need a microscope to see them. In order for us to grow, these cells have to divide. A cell divides into two new cells. Each of these new cells is just like their "mother cell" except for being smaller. Then they grow and divide.

Growth hormone is a chemical that your brain makes to tell other parts of your body to grow.

It's pretty easy for things like skin to get bigger, but things are more complicated in other parts of the body. Bones have to be strong and have a certain shape to do their jobs. (Try to get a bone from a chicken drumstick and take a look at it.) The long middle part is one piece. There are also two end pieces. I'm not talking about the softer white stuff on the very ends. That is cartilage. In an adult chicken or person, these three pieces have fused together into one piece. In a growing person or chicken, they are separate and the place where they meet is softer, like the cartilage caps on the ends of the bone. This is the growth plate. Cells divide here to make the bones longer. As children start to turn into adults, this plate fills in with bone and the bones can't get any longer, so a person stops getting taller. It happens sooner in girls than in boys, so this is one reason men tend to be taller than women.

Some cells don't divide. Brain and other nerve cells hardly divide at all after you are born. Brain cells can make new connections, but you get almost never get new ones. That's why babies have such big heads compared to the size of their bodies.

If we knew how to get cells to divide whenever we wanted them to, we could fix all kinds of injuries. Unfortunately, the signal to stop dividing seems to be in the DNA of each cell and we don't know how to safely re-start the dividing. When cells divide out of control, that's cancer, which is very dangerous. So we have to be very careful about trying to make cells divide after they stop.

If an adult takes growth hormone, it will make the flat bones in their face and hands get wider, but it will not make them any taller. Can you figure out why? Thanks for asking.

A simple answer to this question is cell division. When we are still growing the cells in our body multiply in response to cellular and chemical signals. These signals are carefully controlled throughout our lives, especially when we are only weeks old. For example, when you grow arms in the womb, cells divide an EXACT number of times and then all stop at the same point. If every cell in your new arms divided one time too many, your arm would reach your toes while you were standing up! Amazing!

A person's body is composed of cells, and cells divide to make more cells. Cells are induced to multiply by the circulation of growth hormones in the body, and the expression of genes that induce growth and multiplication. The exact mechanism by which the control occurs is still being worked on.