When you place a stack of banknotes in the counter’s hopper, the bottom note activates a feed-in roller. As the roller rotates, it feeds the bottom banknote to a second roller rotating at a faster speed. Low-friction flanges on each side of this second roller make sure only one bill gets fed in at a time. Next, fanning wheels separate the notes before they pass by an optical sensor that tallies each one (and, depending on the machine’s features, its value). The counted bills then fall neatly into the output stacker on the front of the machine.