The seventh grade science class wrapped up their study of cellular respiration with some practical application in PE class. Students were challenged to experience lactic acid fermentation first hand by performing as many burpees as fast as they could in two minutes and by completing a 90 degree wall sit for as long as possible. As they began each exercise, they felt great: their muscles drew upon stored-up energy to contract. However, once the stores were spent, continuing to exercise at a maximal level, no matter how quickly they breathed, their muscle cells used the oxygen faster than it could be replaced. Consequently, their muscles broke down glucose without using oxygen to get energy, producing lactic acid. Students described feeling “horrible,” “miserable,” “wobbly” and “achy.” Yet, warming down gave their cells more oxygen, broke down the lactic acid, and the pain went away!