Search engines can't just look around the entire internet when you search for something (that would be WAY too slow even if it was possible). URLs have to be sent to it, and from those URLs it finds other sites. It uses its "spider" programs to find out which sites are still there, and what's on them, and that information is actually what's being searched when you search on google or yahoo or anything. Sites with lots of visitors are "crawled' more often, which is why sometimes you find outdated pages & fan sites if you search on google.
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