Inside my parents’ old photo album is a picture of a young boy. He has a round face, freckles, and straight light-blond hair. He loves cartoons, hates avocado, and owns just one record, by Abba. Also inside that album are pictures of a teenager. His face is long, not round; his hair is wavy, not straight. He has no freckles, likes avocado, watches movies rather than cartoons, and would never admit to owning an Abba record! The boy and the teenager are little alike. According to science they have different skin, teeth, blood, and bones. And yet they are both…