system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: if the cells in our body are completely replaced every few years, how do tattoos stay on the body?
gpt
Essentially, when someone gets a tattoo, ink is injected deep into the skin. It goes below the epidermis, which is the layer of skin where cells are constantly growing and sloughing off, and into the dermis, where things are more stable. Cells do get replaced in the dermis, but dead cells are not sloughed off like in the epidermis. Rather, the remnants of dead cells are eaten up by other cells. 

In other words, what's in the dermis will stay there unless removed by cells. Tattoo ink cannot be eaten or broken down by the body's cells, so it will sit in the dermis forever.