Window screens cover openings in homes everywhere. Their fine mesh blocks insects while allowing breezes to pass through.
Each time a window stands open, tiny debris particles enter the air nearby. These include bits of pollen, small leaf fragments, fine threads from plants, and specks of outdoor dirt. They brush against the screen wires and cling there, gathering most in the corners where the frame meets the mesh.
Day after day, with every gust or open hour, a few more particles settle and stick. They layer onto those already present, building thickness without fanfare.
In those corners now sits a noticeable fuzz of material. The screen wires there carry more particles than in the open center areas. This collection grows steadily from the repeated arrivals.
The screens still let air flow freely. Yet the corners hold this ongoing gathering of tiny debris, added to bit by bit over time.