You can each earn some extra credit on your term paper. You get to choose whether you want 2 points added to your grade, or 6 points. But there’s a catch: if more than 10% of the class selects 6 points, then no one gets any points.

Source: http://www.washingtonpost.com/posteverything/wp/2015/07/20/why-i-give-my-students-a-tragedy-of-the-commons-extra-credit-challenge/