I watch my 17-year-old as she goes about her home life, her smartphone cradled in her palm like an extra, permanent appendage while she eats, watches TV, sits chatting with her friend or responds (distractedly) to a conversational overture from me. She’s rarely not looking down at and acting upon it in some way, whether scanning Facebook, posting a quick shot of her cat on Instagram or Snapchat, or playing an inexhaustible supply of games adapted to tiny screens with all the engineering expertise Silicon Valley’s finest have been able to bring to bear on the matter. I worry about…