"There are two kinds of stones, as everyone knows, one of which rolls"

In this quote, Amelia Earhart is describing the various types of stones that exist in the world. She states that there are 2 distinct kinds of stones, and this prevails understanding to everybody. However, she goes on to discuss that a person of these types of stones has the ability to roll. This could be analyzed as a metaphor for people, as some are more passive and go with the flow, while others are more active and take control of their own path. Earhart, being a pioneering aviator, may be indicating that it is important to be like the rolling stone and take charge of one's own destiny.