In our cover story, Faith Salie checks out the science behind yawning. Also: Tracy Smith sits down with Benedict Cumberbatch to discuss his new movie, "The Power of the Dog"; Lee Cowan looks at a promising new treatment for depression; Jim Axelrod interviews sportscaster Bob Costas; Ben Tracy studies the "lithium gold rush"; Hua Hsu talks with Terence Blanchard about his new opera, the first by a Black composer ever staged at New York's Metropolitan Opera; and John Dickerson delves into the biography of Polly Adler, a Prohibition Era madam.