Answer: "Flies have little hairs all over their body that they use to basically see and feel their way around the world. Because these hairs are so small, and the flies are always moving through the air, they get dirty easily making it hard for the fly to “see.” When flies land, they will rub all the dirt off of their hairs and brush them off their hands so they can sense the world more clearly again. "

@Charliedapig