Why do people engage in citizen science projects? The aim of this contribution is to explore the social mechanisms that push nonexperts (i.e., citizens) to invest energy, time, and (sometimes) money in collaborative initiatives on the ground of scientific research. Some relevant examples from the domain of community-based rain measuring are scrutinized, merging the views of a water scientist and a social scientist. After briefly discussing the limits of outdated approaches to science-technology-society issues, social identity theory and new media mechanisms are analyzed as key variables to understand what is new in today's science coming across citizenship. A discussion on the importance of accounting for the uncertainty inherent with the observations coming from crowdsourcing initiatives, possibly the most challenging side effect of what we call Citizen Science 2.0, closes the paper.    For further resources related to this article, please visit the WIREs website.