Look at the middle of your body, and there is your belly button. It is the spot where your cord joined you before you were born. The cord joined you to a placenta, and it was a lifeline. At full term it was about 22 inches long.

Before you were born, you did not eat with a mouth or breathe air. Food and air reached you through the cord. Blood rich in oxygen and nutrients flowed from the placenta to you, and that is how you grew.

When you were born, you breathed air with your lungs, so the cord was not needed for air then. The cord was cut, and a short piece of it stayed on your body. That short piece is called the stump.

The stump dries up and falls off in one to three weeks. As it dries, it changes color. At first it is soft and moist, and it is blue white or yellow green. Then it turns yellow, and last it turns brown or black.

When the stump falls off, it leaves a dent behind. That dent is called the navel. A navel looks like a pushed-in place with a rim around it. Navel is another name for the belly button. Doctors call it the umbilicus, too.

A belly button is a scar. It is left over from the cord. It is your very first scar. After the cord is cut, the opening inside it heals and seals itself shut, and the scar stays on your body.

An innie is a little dent, and an outie sticks out. Most belly buttons are innies. Outies are fewer. Which one you get may depend on how the skin heals after the stump falls off. Nobody gets to choose. It is mostly luck.

Furry mammals have belly buttons too, but the small flat scar is usually hidden by their hair, so you cannot see it. Platypuses are different, because they lay eggs, so they have no umbilical cord and no belly button at all.

Words to know

• stump: bit of cord
• navel: belly button
• platypuses: egg-laying animals
• placenta: cord's other end