Wikipedia:Tip of the day/February 28
It is important that the information in Wikipedia be true and accurate. To help ensure this, we have the Verifiability Policy, which states that all facts presented on Wikipedia must be verifiable.
When writing facts, 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 for a particular detail.