system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If over a period of time we lose and replace each cell in our bodies, how do tattoos stay imprinted on us?

Hope that makes sense. I was watching a Vsauce episode and he mentioned that over the course of about 5 years, each cell in your body is lost and replaced. How do our new cells remember ink when they are replaced?
gpt
The ink molecules in tattoos are too large for the body to flush away. It's buried deep enough that the ink isn't shed along with the top layers of skin, but the body also can't break down the ink to remove the ink through internal body processes.

The reason tattoos fade over time when exposed to sun is that the UV light breaks apart some of the ink molecules, making them small enough for the body to remove. Professional tattoo removal is using specific frequencies of UV light to more efficiently and more quickly break down the ink depending on the color and ink composition. Some colors of ink are more difficult to remove than others.