Playfulness is what makes us human. Doing pointless, purposeless things, just for fun. Doing things for the sheer devilment of it. Being silly for the sake of being silly. Larking around. Taking pleasure in activities that do not advantage us and have nothing to do with our survival. These are the highest signs of intelligence. It is when a creature, having met and surmounted all the practical needs that face him, decides to dance that we know we are in the presence of a human. It is when a creature, having successfully performed all necessary functions, starts to play the fool, just for the hell of it, that we know he is not a robot.
http://www.newstatesman.com/2013/10/plays-thing