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This automatically limits your search to a single source.
You're also given the option to run a search on those specific terms.
Note: In some cases the query will not auto rewrite itself but an option to limit to source is available. For example: a search for the keyword train in The Guardian.
And if you know the news source, you can just type it first. So
cricmania cricket
will get rewritten to
source:cricmania cricket
That let's people restrict their searches without memorizing syntax, but we give them power hints so that they can do a more advanced search next time if they wish.