Methods, systems, and apparatus, including computer programs encoded on computer storage media, for collecting user interaction data of a plurality of users for each of a first plurality of document-text pairs, wherein the user interaction data is collected for the document-text pair from a respective service for which the respective text of the document-text pair was selected. A respective weight is calculated for each of the first plurality of document-text pairs based on, at least, the collected user interaction data for the document-text pair. One or more topics are associated with one or more of the sources based on, at least, the respective weights associated with a plurality of first document-text pairs that are associated with the source.