In the popular “ Little Bit of” series: a fresh, accessible introduction to the postures and practice of yoga.     Although it is an age-old practice originating in India, yoga is still embraced by many as a means of enhancing physical, mental, and spiritual health. Not only does it calm and strengthen, but yoga can improve flexibility and mental clarity through deep breathing and poses. This accessible introduction presents the history and philosophy of the form, along with basic poses—including Adho Mukha Svanasana/“Downward Facing Dog,” Virabhadrasana/“Warrior,” and Vrksasana/“Tree”—and advice on incorporating them into a daily exercise program. An introduction to postures and practice.