The balls can be transformed on boxes, and these in balls. Use this fact to eliminate all the red ones, without losing the others! You play by converting boxes in balls when you want them to roll so they push other boxes and balls, and also convert balls in boxes when you want them to stop rolling, with the goal of moving out of screen the red balls, but keeping the rest. Some red boxes cannot be transformed, in this case you'll have to push them as boxes.
Click on a box to transform it in a ball, and click on a ball to transform it in a box.