system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do ceiling fans collect dust while the fan is on?
gpt
The surface of the blades isn't entirely without friction, and dust comes in different sizes and forms. As the blades turn, dust small enough to fit gets trapped onto small cavities, ridges and hooks of this surface. The more dust that gets stuck this way the easier it becomes for other bits of dust to get stuck to the existing bits, creating layers. Everything that moves also build up a charge of static electricity, which makes matter such as dust cling to it easier. 

The compact layers of dust that forms on them is why it appears so dark when wiping fan-blades.