system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How does so much dust accumulate on fans? why isn't it just thrown off?
gpt
Fans are electrically powered moving surfaces and so they contact a lot of dust in the air, way more than a non-moving surface like a wall . That movement also helps them build up a static charge that causes dust to sometimes stick to them harder than the wind that they generate can blow off. Eventually that dust can built up into a bit of a mat that isn't really smooth, and that just makes it easier for more dust that it encounters to lodge itself into place.

And fans that might be close to a kitchen or really dirty area might also hit suspended oil droplets from that fish someone fried, or other forms of slightly greasy dust that act like glue and really cement stuff on.