Cross-train your brain. All it takes is ten to fifteen minutes a day of playing the right games. (It’s fun.)Exercising your brain is like exercising your body—with the right program, you can keep your brain young, strong, agile, and adaptable. Organized on an increasing scale of difficulty from “Warm-up” to “Merciless,” here are 399 puzzles, trivia quizzes, brainteasers, and word game that are both fun and engaging to play, and are expertly designed to give your brain the kind of workout that stimulates neurogenesis, the process of rejuvenating the brain by growing new brain cells.