College yoga instructor: "You sit wrong. What you're doing is not sitting up straight. Go home and practice sitting."

Reddit comment: "When the knees are higher than the hips, shortness in the back of the pelvis or the hip joints tips the pelvis posteriorly and tips the weight of the body backward. To maintain balance and prevent falling, the spine rounds into flexion. When the hips are raised higher than the knees, the pelvis tips anteriorly and the weight tips forward. This allows for..."