1. A method, comprising: generating a dictionary comprising (a) one or more transformations of words from singular to plural form and (b) one or more transformations of words from plural to singular form; counting a first number of times that a certain word appears in a set of two or more separate web documents, wherein the certain word is a singular form of the word; counting a second number of times that a transformation of the certain word appears in the set of two or more separate web documents, wherein the transformation of the certain word is a plural form of the certain word; computing a probability by either (a) dividing the second number by the first number or (b) dividing the first number by the second number, removing, from the dictionary, the certain word and the transformation of the certain word in response to determining that the probability is below a particular value; receiving a particular query from a user comprising a plurality of words; based on the particular query, determining whether a particular word in the particular query occurs in the dictionary; selecting from the dictionary a transformation of the particular word; and generating results based on executing a version of the particular query that includes both the particular word and the transformation of the particular word; wherein the method is performed by one or more computing devices.