Roller mop

Most cheap robot mops are a wet rag clipped to the bottom of the robot. A roller mop is a rotating cloth drum, usually spinning a couple of hundred times a minute, that presses down on the floor and scrubs.

The practical difference shows up with dried or sticky messes. In Gizmodo’s test the roller-equipped Roomba Max 775 cleaned up a puddle of ketchup rather than smearing it across the floor, which flat-pad robots often do.

The catch: a roller has to be washed constantly or it just spreads dirty water, which is why roller-mop robots come with big docks that rinse the roller with hot water mid-clean.