The notion that yoga doesn't involve sweating, pain or physical exertion is one held only by people who have never been on a yoga mat trying to balance their body weight on their left heel, the ball of their right foot and their left hand gripping a yoga brick while also finding their inner peace. It's like Twister for the body and the soul. Indeed this quote from the official BKS Iyengar Web site--"Pain comes to guide you. Pain is your guru"--sounds more like something that requires sturdy shoulder pads instead of a thick floor mat. Beginning Wednesday and continuing through May 12, Lisa Waters and Michelle Mock will teach the basic asanas used in Iyengar yoga at the Crow Collection of Asian Art, 2010 Flora St. Classes are 5:30 p.m. to 6:30 p.m. Tuition is $15 per class or $80 for the entire session. Friends of the Crow Collection are admitted for free. Reservations are required. Call 214-979-6438.