How much of that are you tracking in? Tracking it in implies that it comes off your shoes wherever you walk. But that also goes for everywhere you walk. So i

How much of that are you tracking in? Tracking it in implies that it comes off your shoes wherever you walk. But that also goes for everywhere you walk. So in reality you are only tracking in whatever disgusting stuff was within the last X steps of your house. If you walk on the sidewalk, and if it rains periodically, it gets constantly cleaned. Walking on grass, well you are walking on something that will really pull gunk off your shoes, and it is has enough bio-stuff there that breaks everything down.