This week we've got a question from Lulu Miller, the host of NPR's Invisibilia podcast:

How old are our bodies actually?

Because if you think about how we're constantly being regenerated our physical matter is being shed and replenished... what actually remains?

Good question, Lulu!

Well, your hair obviously doesn't remain the same and I'm not just talking about the style.

Each hair on your head is replaced every 2 to 7 years.

A hundred hairs fall out every day and new ones grow back in their place.

And look at your fingernails - they're completely new every six months or so.

It turns out it's just a matter of time before almost every part of your body refreshes itself in a similar way.

The lining of your stomach and intestines gets pretty beat up -- it's constantly exposed to acid and bile and so those cells get replaced every few days.

Every few weeks, your outer layer of skin is completely renewed.

Every four months, you have a fresh army of red blood cells.

A hundred million new cells are born every minute and a hundred million old cells are destroyed.

It's actually the breakdown products of these red blood cells that turn your bruises and urine yellow.

Every 10 years, you've got a new skeleton.

A special team of cells breaks down old bone, and another builds new bone.

Every 15 years, your muscles are refreshed.

You might think you gain and lose fat cells when you gain and lose weight but they actually just get bigger and smaller.

Over the course of 25 years though, most of them turn over.

But there are a few things that stick around for your entire life.

About half of your heart stays with you from birth to death because those cells are replaced very slowly.

Certain parts of your brain add a few new neurons over the course of your life, but the vast majority of your neurons developed before you were born.

It's the connections between those neurons -- the circuits that store memories that are constantly changing.

And there's one more part of you that lasts your whole life.

Months before you were born, a little cluster of cells stretched and filled themselves with transparent protein.

As you grew, even after birth, more and more fibers were added, but that center endured.

This is your lens, the window through which you are watching this video right now, and its core has remained the same since the moment you first opened your eyes.

So how old is your body really?

Some of it is brand-new and some of it is as old as you are.

To learn how scientists figured all this out, check out our post at skunkbear.tumblr.com

And listen to Invisibilia -- the latest episode is about whether or not personality persists throughout your entire life

If you have science questions send them our way and please subscribe to our channel!