In one embodiment, a computing device receives text input from a user. The computing device determines an action by the user indicated in the text, and at least a part of a phrase in the text that indicates one or more topics associated with the action. The computing device determines whether the at least part of a phrase matches one or more phrases stored in an indexed database. When the at least part of a phrase does not match one or more phrases of the indexed database, then the computing device determines a complete phrase input by the user, and determines one or more topics to be associated with the complete phrase. The computing device stores the complete phrase as an instance of the complete phrase. The computing device determines whether at least a threshold number of instances of the complete phrase has been stored.