10. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a first word and a second word; computing a frequency with which the first word occurs in a collection of documents and a frequency with which the second word occurs in the collection of documents; determining that the frequency with which the first word occurs in the collection of documents is greater than the frequency with which the second word occurs in the collection of documents by at least a certain amount; and determining that the pair of words are not synonyms based at least on determining that the frequency with which the first word occurs in the collection of documents is greater than the frequency with which the second word occurs in the collection of documents by at least the certain amount.
10. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a first word and a second word; computing a frequency with which the first word occurs in a collection of documents and a frequency with which the second word occurs in the collection of documents; determining that the frequency with which the first word occurs in the collection of documents is greater than the frequency with which the second word occurs in the collection of documents by at least a certain amount; and determining that the pair of words are not synonyms based at least on determining that the frequency with which the first word occurs in the collection of documents is greater than the frequency with which the second word occurs in the collection of documents by at least the certain amount. 11. The system of claim 10 , wherein the operations further comprise: excluding the pair of words from a list of synonyms used to generate alternative search queries.