Dear David,

When I was a kitten, I used to keep track of my growth. Every now and then, I’d make a little pencil mark on the wall right above my ears.

We might not be growing taller every second, but parts of us do grow all the time. We grow new hair. We grow new fingernails. We grow new bone. We even grow new skin.

My friend Jonathan Jones, a scientist and professor here at Washington State University, is really curious about skin. His research helps us learn more about how our body helps heal wounds.

Skin is our body’s biggest organ, he said. It helps protect us. While you can’t see your individual cells without a microscope, your body is actually growing new ones at this very moment. And at this moment, too.

“As long as we are alive, our cells turn over,” Jones said. “I guess you could say that you turn over, or at least replace, what cells you have already.”

Every 40 days or so you’ll get a new layer of skin. Babies only take about 15 days to grow a new layer skin. I asked Jones why babies can grow new skin cells so much faster.

“We don’t know,” he said. “We would love to know why. As cells age, they get more problems. They don’t turn over as fast.”

Our bone cells have also been growing since we were babies. Our bone-building cells and bone-eating cells work away on our skeleton. In a way, our skeletal system is always remaking itself.

When you were about two years old, your brain nearly reached the size it would be for the rest of your life. We are born with tons of brain cells that will communicate with the rest of our body. For a while, scientists weren’t sure grown-ups could grow new brain cells. But it turns out they actually can grow new brain cells.

Meanwhile, other kinds of cells in our body grow pretty quickly, too. The ones in our stomach lining, for example. Our stomachs have a protective lining that we replace every five days or so. It helps keep our stomach from digesting itself.

Come to think of it, we would take up a lot of space if all our parts were literally getting bigger and growing every second. It might be a little awkward if we never stopped growing physically—if our body kept taking on new cells without getting rid of old ones.

But even if we stop growing taller, our bodies are still growing in all kinds of other ways. After all, a little more than a month from now, you’ll be in a whole new skin.

Dr. Universe