1. Introduction
During these last ten years, thanks to the tremendous performance improvement of consumer-level hardware, we have been witnessing a rapid growth in the video game market. Video games, once limited to pure entertainment and storytelling, are now trying to propose new experiences and to communicate at an emotional level [1]–[3]. Among these new generation of games we can spot a branch named serious games. The term “serious game” is usually adopted to indicate a game where the user's experience is strictly related with a formative training like scientific exploration, healthcare, emergency management, city planning, engineering, religion, and politics [4]–[7]. However, this definition is nowadays outdated as serious games are not anymore solely those involved in training. Instead, they have reached new challenging realms providing the possibility to achieve an important (serious) outcome through the fun and participative paradigm of games. As an example, think to Google Image Labeler, which exploits the effort of a multitude of human players to create an immense database of tagged image [8]; the rationale lies in the fact that appropriately tagging the content of an image can be easily done by humans while it is a currently an impossible task for a computer.