As part of their research on the future of play, RMIT University's Exertion Games Lab demonstrated a game based around an ingestible sensor pill that measures internal body temperature and transmits the data in real-time to your smartphone as the sensors travels through your gut. You can guess what marks the end of the game. (This is the same group that explored the use of chest-mounted robot arms for "playful eating.") From their research paper (PDF):
In the Guts Game, a chocolate bar is given to each player initially. Then the researchers, who dress up like medical doctors, tell the players that they have been infected by a parasite, which is sensitive to its environment's temperature, i.e. the body temperature of the player as measured by the sensor. If the environment's temperature reaches a certain value, the parasite will be hurt. The crafty parasite may adapt to the environment so the target temperature might change once reached. The more often the player reaches the target temperature, the bigger possibility he/she will survive. To aid the treatment, the "doctors" developed an application called the Guts Game. Players need to swallow the sensor to measure their body temperature and the application will guide players. Players need to come back to the "doctors" after the game ends to check if the parasite is still there.
The Guts Game ends when one of the players excretes the sensor.
The Guts Game (Exertion Games Lab)