Our research focuses on developing elementary students' mental computation skills with the help of card games. Choosing this area of study was motivated by our personal experiences, namely, that mathematics programmes of study do not lay emphasis on this aspect; there are too few hours dedicated to developing this skill, and several mental computation strategies are not presented in the classroom. As a result, students struggle with mental computation, a basic skill needed in our day-to-day life, and a relevant basis for further mathematical knowledge, computation competence, and higher order thinking skills. In order to develop this skill, we have transformed well-known card games into didactic games, which encourage development almost unnoticeably, given the fact that children love to play, they need play. We have devised five new, appealing tools, with different levels of difficulty, suitable for developing mental computation skills in an attractive way. This paper presents an experiment involving second grade (8-9 years old) students. Two experimental groups were involved. The aim was to investigate whether mental computation is more efficient when using the traditional method or when using card games. We hypothesized that using card games might prove helpful in developing mental computation skills.