Mostly girls do it. They try to smell their own dry stinky saliva from the lips. This smell not exactly works as a pheromones bit its a big turn on for them. It is done pouting lips together and touching upper lip to the tip of a nose so that smell directly enters into nostrils. Most of girls frequently suck their lips inside their mouth just to make a coating of stinky saliva on the lips so that it will produce nice smelling effect once it gets dry. The other reason is just to block the nostrils and smell inside sidewalls of the nose. It also turns them on.

When you eat dirt for the first time and leave your family to build a shack in the woods