This is a great activity about human energy production. Learners will work out with a clothespin to investigate why hockey players jump on a stationary bike after an intense game. Compared to a minute of ice time in an NHL game, this clothespin activity may seem like a "wimpy" workout—but it does a great job of demonstrating aerobic and anaerobic exercise and explaining the production of lactic acid, the cause of sore muscles after a workout.