 | This help page is a how-to guide. It explains concepts or processes used by the Wikipedia community. It is not one of Wikipedia's policies or guidelines, and may reflect varying levels of consensus. |
 | This page in a nutshell: Sources are rarely plundered for all they are worth, and articles with "citation needed" tags often already have sufficient sources that simply have been under-utilized. Most new sources added for a detail or two can also be dug into for additional sourcing value. |
It is very common for Wikipedia editors to add a citation, such as to a newspaper or magazine article, a book chapter, or other hopefully reliable publication, to source the verifiability of a single fact in an article. Most often the editor has found this source via a search engine, or perhaps even a library visit, seeking a source for a detail in an article, some pesky tidbit without a citation. This common approach tends to miss many opportunities to improve both the content and the sourcing of articles; it's akin to stopping at a grocery store for bread and nothing else, rather than "working" the store for an hour with a long shopping list and an eye for bargains.
This tutorial offers a very short but real-world example of how to "mine" a source– to really work it like a pocket of ore for every last bit of verifiability gold. In addition to noticing facts in your source that are missing from the article, and noticing that your source can also provide a citation for more facts already in the article than the one(s) you were most concerned about, you can also often double-up citations on a fact that already has one source cited. While the average fact in an article does not need seven citations, having two rarely hurts, can provide a cushion if something is found faulty with the other source and it is deemed unreliable or its link goes dead, and can provide backup sourcing if a third, questionable, source challenges the first.