Malzahn, Nils, Sabrina Ziebarth, Dominik Kloke, and H. Ulrich Hoppe. “Self-Enriching Ontology-Based Casual Learning Games”. COMPUTING AND INFORMATICS 34, no. 3 (October 24, 2015): 652–677. Accessed October 28, 2021. http://www.cai.sk/ojs/index.php/cai/article/view/3224.