Kindergartners MaKenna Bailey and Chloe Pike stretch at J. B. Stephens Elementary School in Greenfield, Ind., Wednesday. A new study found that kindergartners and first-graders' body mass indexes grow at a faster rate during summer vacation than during the school year. The findings suggest that children gain more fat mass during the summer, but that kids' busy school days help them stay fit. (AP Photo/Tom Strickland) Tom Strickland