In "The Young and Prodigious T.S. Spivet," the running-away letter 10-year-old T.S. Spivet (Kyle Catlett) leaves for his parents has got to be one of the most moving epistles in cinema history. Brimming with modesty, formal but warmly polite, it could have been written by a nice Japanese son. "I have gone for a while to do some work," is the opening sentence, and the letter ends: "You are one of the best families in the world."