system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When someone rubs their fingers together rather forcefully for a while, why does black residue start to appear?
gpt
The friction causes your epithelial cells to slough off and the oils combined with sweat and dust and whatever else you might have on your finger (or in the grooves of) accumulate into the dirty looking matter you're asking about.

If in the shower, you can also do this on your body. Some places (like the behind the ankle) are easier than others.