Fight the cold of winter. Warm your muscles from the inside out with a yoga session. Try a single class at Bikram Yoga on Lincoln Street at $20 for adults, $15 for students or seniors, or $10 for the early bird special. Maybe you want to kick it up a notch with a more challenging yoga experience. Try a walk-in class at The Yoga Studio. Ninety-minute classes are $16, and a two-hour session is $20. Just want to test the waters first? Sadhana in the South End offers its first class for free.