Men's Restroom or Women's? Nature Is Never That Simple

From preschool onward, children learn to categorize everything -- rocks, animals, plants and, of course, people. Eventually we learn to box our fellow humans into categories based on skin color, language, religion, personality type and perhaps most basic of all, sex.

The problem with these mental shortcuts is that the world is full of exceptions that blur the boundaries. Nature can be creative in producing intermediate cases.