Robbi Giuliano teaches her fifth grade class as they sit on yoga balls at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. By making the sitter work to stay balanced, the balls force muscle engagement and increased blood flow, leading to more alertness. (AP Photo/Matt Rourke)

Associated Press
Robbi Giuliano teaches her fifth grade class as they sit on yoga balls at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa.   By making the sitter work to stay balanced, the balls force muscle engagement and increased blood flow, leading to more alertness.  (AP Photo/Matt Rourke)
Robbi Giuliano teaches her fifth grade class as they sit on yoga balls at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. By making the sitter work to stay balanced, the balls force muscle engagement and increased blood flow, leading to more alertness. (AP Photo/Matt Rourke)
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