Book excerpt: For going on two decades, Scientific American's "Ask the Experts" column has been answering reader questions on all fields of science. We've taken your questions from the basic to the esoteric and reached out to top scientists, professors and researchers to find out why the sky is blue or how planets acquire rings. Now, we've combed through our archives and have compiled some of the most interesting questions (and answers) into a series of eBooks. Organized by subject, each eBook provides short, easily digestible answers to questions on that particular branch of the sciences. The Human Body and Mind is the third eBook in this series, and it tackles questions about our own strange and mysterious biology. [Note: Health and medicine will be covered in a separate eBook.] Our experts field queries on evolution, bodily quirks and psychological feats. Have you ever wondered why humans lost their body hair? Curious about what causes a hangover? Or what makes that popping sound when we crack our knuckles? What about the oft-cited maxim that we only use 10 percent of our brains? Professors, scientists and biologists provide answers that are at once accurate, understandable and sometimes just plain funny.