7. The method of claim 1 , further comprising determining the limited amount of time based on a frequency of co-occurrence of the two or more identified keywords in the set of text sources.
1. A method performed by data processing apparatus, the method comprising: identifying, in a set of text sources that were previously published within a specified amount of time of a current time, two or more keywords that co-occur in at least one of the text sources; determining that the two or more identified keywords are not designated as co-occurring keywords in a data structure storing historically linked co-occurring keywords; in response to determining that the two or more identified keywords are not designated as co-occurring keywords and in response to the two or more identified keywords co-occurring in at least one of the text sources, temporarily linking the two or more identified keywords for a limited amount of time; and distributing, during the limited amount of time, content items based on the temporary link between the two or more identified keywords.