1. Raymer, R.: Gamification: Using Game Mechanics to Enhance eLearning (September 2011),
http://elearnmag.acm.org/featured.cfm?aid=2031772#.T1KMoq8mZPY.twitter
2. Chatfield, T.: 7 ways games reward the brain. TED Global (July 2010),
http://www.ted.com/talks/tom_chatfield_7_ways_games_reward_the_brain.html
3. Kittle, M.: Motivating Employees: Gamification at Work. VP Digital Strategy, Bunchball,
http://schedule.sxsw.com/2012/events/event_IAP13069
4. Julia: Make It a Game: Using Gamification to Build Your Business. Small Businesses (January 18, 2012),
https://www.odesk.com/blog/2012/01/make-it-a-game-using-gamification-to-build-your-business/
5. Game Mechanics. Gamification Wiki,
http://gamification.org/wiki/Game_Mechanics#Achievements