Play involves unstructured activity that is freely entered into and intrinsically rewarding. When children engage in play there is little intentional learning. There are no lesson plans. There are no daily objectives or specific learning outcomes. Incidental learning can cause relatively permanent changes in the way one thinks; accidental learning occurs as a result of experience. The following information focuses on the incidental learning of mechanical principles that occurs on playgrounds. The mechanical principles are derived from physics and deal with objects, forces, and movement. In this article, the author discusses age-appropriate biomechanics and Newton's Law of Action-Reaction through mechanical devices that can be seen in the playgrounds such as swing, slide, etc.