During the last years there is an enhanced educational and research interest about the second generation services and applications in the Web (Web 2.0) and, especially, about weblogs (blogs) as tools for social construction of knowledge. A study, concerning secondary education students' use of an educational blog aiming at sharing ideas and debating about the issue of athletes' doping, is presented in this paper. The findings regarding the analysis of the students' cognitive presence show that the educational blogs can be an effective tool supporting collaboration and scientific knowledge construction.