How do you girls show sign of interest to a stranger?

I don't mean to come off as arrogant or anything, but sometimes I pick up strange signals from girls. Their behavior seems noticeable as it changes, and sometimes I just don't respond but just watch them or keep going my way. Theres one that seems to become a bit more graceful in her way of moving when she sees me walk into her workplace. Sometimes others twirl their hair after I make a joke, but its weird when it happens because it is with the girls that I least expect it with....