Wikipedia:Tip of the day/August 28
It is important that the information in Wikipedia be factual; that is, true and accurate. To ensure that this is the case, one of Wikipedia's core policies is to verify all information being presented.
Therefore, when writing, it is very important that you cite your sources! Stick to a factual style even when writing about fiction or opinions. Providing references is particularly important for controversial opinions - instead of using weasel words like some people believe... you should cite sources to help readers learn who believes what, and why.
Keep in mind that the folks at Wikipedia's Reference Desk are very knowledgeable across a wide range of subjects, and they may be able to help you track down source references to a particular detail.