This page in a nutshell: Do not use a Wikipedia article as a source for another Wikipedia article, except when describing Wikipedia.
Wikipedia is not a reliable source. It generally uses reliable secondary sources, which vet data from primary sources. If the information on another Wikipedia page (which you want to cite as the source) has a primary or secondary source, you should be able to cite that primary or secondary source and eliminate the middleman (or middlepage in this case). Always be careful of what you read; it may not consistently be reliable because work submitted to Wikipedia can be edited, used, and redistributed by anyone. As indicated by WP:CIRCULAR, which is part of the Verifiability policy, neither articles on Wikipedia nor websites that mirror Wikipedia can be used as sources, because this is circular sourcing. An exception to this is when Wikipedia is being discussed in an article, which may cite an article, guideline, discussion, statistic or other content from Wikipedia or a sister project as a primary source to support a statement about Wikipedia (while avoiding undue emphasis on Wikipedia's role or views, and inappropriate self-reference).
