Many people do not get the recommended 30 minutes of exercise per day, which can result in health problems such as obesity, muscle atrophy and chronic disease. Based on the principles of casual games and exergames, we propose and define casual exergames for motivating people to exercise in multiple small chunks of time throughout the day. We designed, implemented, and tested a casual exergame called GrabApple. Our preliminary results show that users enjoyed playing the casual exergame and that in just 10 minutes of play, their heart rate was elevated to an average of 72% of maximum heart rate, and that they burned an average of 91.8 Calories. If played three times per day, our casual exergame produced sufficient physical activity to meet current fitness guidelines. We discuss the potential health benefits of casual exergames.