For the past five years, our students have been passing less and less time preparing for lectures and exams. To encourage them to do more exercises, a pedagogical activity was offered outside the classroom. With the goal of making students more active during the problem-solving process, an innovative online environment, Sphinx, was developed. Sphinx proposes a set of exercises with their solutions, and invites students to explain them. Sphinx also gives students the opportunity to exchange ideas about an exercise and its solution, and gives the teacher the ability to observe if students take part in the problem-solving process. The originality of Sphinx lies in its ability to support students between lectures with a common learning strategy: learning-by-example. An experiment was conducted with 137 students during the 2003 fall session. The solved exercises were consulted most often before the two exams. However, only a few students participated actively in the experiment by intensively using the Sphinx environment; they obtained a better average score than the rest of the class. Thus, participation and collaboration had a positive effect on the students' marks. Tools should be added to motivate more students to take part in the explaining and collaborating process. (Contains 2 tables and 3 figures.)