In this workshop we will try to discuss the question: how can a computer science contest contribute to knowledge construction? Usually a contest tests already learned knowledge and skills. In the case of the Bebras contest no pre-knowledge and no specific skills are required by the students (age 10-19). While participating in the contest they can learn about different aspects of computer science concepts, like information representation, structures, algorithmic aspects, automation aspects, etc. While trying to solve given tasks the students may construct their knowledge that is related to the presented problem sets. The quality of the problem set is most important for giving the right stimuli for learning. That is why we will discuss, based on some given Bebras tasks, the attributes of tasks that allow a maximum effect on the learner´s knowledge construction.