Today’s question: Does your body really replace itself every 7 years?

The short answer is “no,” but don’t worry: This isn’t a case of chicanerous researchers pulling the wool of shoddy science over your eyes.

Your body mostly replaces itself every 7 to 15 years. Some bits are never replaced. Others, like the lining of your stomach and intestines, are renewed much faster. Due to constant wear and tear from the process of digestion, these cells have an average lifespan of just 5 days!

Yes, the organs that work the hardest have the fastest changeover. You get a whole new skin every 2 to 4 weeks. Your red blood cells last less than half a year – not bad, considering that their route through your circulatory system is about a thousand miles. And your liver renews itself at least once every couple of years. As the human body’s detoxifier, it goes through a lot.

Other tissues take longer to completely replenish themselves. Like your bones. Skeletal cells die and new ones grow constantly, but the complete process takes about 10 years. (And the process slows down as we get older, which is why our bones tend to get weaker as we age.)

And, like I said, some parts of your body stay with you for life. The cells on the inner lens of your eye formed when you were just an embryo. Your tooth enamel wears down with use, never to return.

And evidence indicates that you can’t regrow the neurons of your cerebral cortex. Its loss can lead to diseases like dementia. Luckily, other parts of your brain do regenerate. Like the hippocampus, which helps us create memories, and the olfactory bulb, which helps us smell.

So how do we know all this? Turns out, it’s thanks to our old pal nuclear weapons testing. Yeah! High-fives for radioactive stuff being released into the atmosphere!

No, really: Aboveground nuclear detonations during World War II and the Cold War spiked Earth’s air supply with extra carbon-14. It’s been declining back toward the norm at a predictable rate since the 1960s. Which means that you can use the amount of it present in any given tissue sample to determine when those cells were born. More carbon-14 means older cells.