Why do flies rub their legs together?

A fly’s body and legs are covered with several bristly hairs. Dirt gets trapped in these hairs. By rubbing the legs together, the fly gets rid of the dirt. The dirt gets scraped off when it rubs its legs and thus the fly cleans itself. As flies have a tendency to alight on food, this spells trouble for people as the germs and dirt can get transferred to the food they eat.