Human behaviour is what makes the game agar.io so great. In this game i occasionally observed players cooperating very successfully in free-for-all mode. The challenge is to find another player to cooperate with by winning their trust. You can give each other energy which makes the other play grow (and yourself shrink).

It gets super interesting once the power relationship changes in the game and the formerly weaker player can now kill the other player. Will he/she remain cooperative? Will you be backstabbed? A study in psychology and great fun.