If anyone has ever tried to tell you that you've changed, they're right. Well, probably not for the reasons they think, but on a more technical level. Our bodies are constantly changing as our atoms recycle and as our cells divide. Some of your body parts are just a few weeks or even days old. Just how long the cells in certain areas last depends on how much work they do and the type of environment they are in. For example, your taste buds have an average lifespan of between 10 to 14 days, but environmental factors such as, say, piping hot coffee could shorten that lifespan by quite a bit. As another example, your lungs replace the old cells with new ones every two to three weeks.

The cells lining your stomach exist in a highly acidic environment, and therefore they are replaced about every five days. Red blood cells live for approximately four months in the body before they are replaced. On the other end of the scale, some cells take much longer to regenerate. Your skeleton completely remodels itself and replaces all of its cells every ten years or so. This is roughly the same amount of time it takes for our fat storage cells to completely recycle. Then there are cells in your body that live for many decades, such as the hippocampal neurons in your brain that are replaced every 20 to 30 years. There are also cells that stay with you for a lifetime and do not regenerate. Your permanent teeth are just that, permanent, and they never regrow.

Your eyes are one of the few body parts that don't really change during your lifetime. The only part that is constantly being renewed is the cornea, which is the top transparent layer. Neurons in the cerebral cortex, the brain's outside layer that governs memory, thought, language, attention, and consciousness, stay with us from birth till death. Until recently, it was thought that the heart couldn't renew itself. However, a study showed that our hearts have stem cells that can replace old heart cells with new ones at least three to four times over a lifetime. Obviously, it's good for us that our cells are constantly being recycled. Imagine if the taste buds that you've burnt off with coffee or tea were gone forever. But, as it turns out, this constant recycling of cells is ultimately what leads to our death.

As our cells divide, information in our DNA is slowly lost over time. Everyone is familiar with this concept, but you may know it by another name. Aging. Eventually, cells divide themselves to a point where they can no longer continue, and they start to die. Over time, the dying cells outnumber the dividing cells, and our bodies slowly start to deteriorate on a cellular level. As it turns out, we may be able to prevent this.