In Walden, his 1854 reflection on simple living, Henry David Thoreau wrote, ‘The cost of a thing is the amount of . . . life which is required to be exchanged for it, immediately or in the long run.’ So what was the opportunity cost to children and adolescents when they started spending six, or eight, or perhaps even 16 hours each day interacting with their devices? Might they have exchanged any parts of life that were necessary for healthy human development?