system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do tattoos stay on permanently if the cells in our body are continually replacing each other.
gpt
Cells replace each other one by one. Whenever a cell dies another cell is formed to fill the void left over. So the entire structure of your skin stays largely intact even though the individual cells all gets replaced. The ink that make up the tattoo is firmly lodged between the cells and will therefore not be able to get out past the cells. Even as cells gets replaced they do not leave any gap for the ink to pass though. So the ink stays largely in the same area as all the cells around it gets replaced.

You can sort of compare this to roads in a city. Even though all the buildings gets replaced the roads are left largely in the same place. If you want to move a road you would have to demolish all buildings along the new road all at once which is often not feasable. So a modern road through a city will often follow the exact same path as the old medeval cart path even though all the buildings around the road have been replaced multiple times since.