Because preschoolers are engaging in many gross motor activities for the first time and children's perceptual judgments are still immature a significant amount of direct adult supervision is necessary. The physical environment should have pieces of equipment that vary in skill level according to the degree of balance and coordination required. Under climbing structures, six to twelve inches of appropriate cushioning material is necessary to protect children from falls, a major cause of childhood injury (see AAP & APHA 1992: Kendrick, Kaufmann, & Messenger 1995). Planned outdoor activities should challenge children to use a range of motor skills, as obstacle courses do, but allow for and be adapted to a wide range of difference among children due to maturational rates; motivation level; experience, practice, and adult coaching; nutrition; and identified or potential disabilities and exceptional abilities.