A friend of mine has two remarkable little girls, ages 5 and 8, and they have very different personalities. The youngest is spritely, whimsical and exuberant. The oldest is brilliant, methodical and mightily capable. She taught me how to tie a square knot. She’s wont to say things like, "Tara, I can row from this side of the bay to that one, in a boat the size of a bathtub. By myself." And she can.