The dustpan line. We've all encountered it: that little bit of dust, crumbs, and dirt that lingers behind when you sweep a pile into the dustpan. Maybe you ignore it and turn away in defeat. Maybe you give it a careless brush into the nether when the job is done. Or maybe you sweep it over and over, watching in hopeless fury as each time a new line appears, parallel to the last, slightly diminished but never quite disappearing.