How can people use wikipedia as a source?

it is a source because it has information which some people know and they share it on the internet. it has information on nearly everything everyone knows.

dude, i hate how everyone talks smack anout wikipedia. yeah, everyone can edit it, but if it's wrong, or has no source, people edit it out. in fact, wikipedia is the BEST place to use a source, it's easy to find, it has over 2.6 MILLION articles, and over 99.99999999999% is true. I use wikipedia as a source, because it's edited very regularly. so, my answer is they can because it's a valid source, and many people like it.

Most vandalism is minor. Vandals aren't generally clever or creative enough for sneaky vandalism. When they try it, their grammar and usage is usually so bad that changes are easily spotted. Vandalism is very easy to fix with a simple revert and it's usually caught quickly. The only vandalism of this type that I've seen and had to revert myself is usually no more than a couple days old and that's usually on some obscure topic. The only misinformation I've seen was a death date added to a page based on a rumor. I did a little research to confirm that the person was still alive, went back five minutes later, and it was already reverted. I often use Wikipedia as a source, but I generally don't answer questions I know absolutely nothing about. It's much easier to copy and paste parts of a Wikipedia page than write everything out. If I need another source, Wikipedia is still often my starting point. Most pages are well cited. Go to the bottom of the page and use the references as your own source. I've seen hundreds of people badmouth Wikipedia, but I've never seen ONE substantiate it with a SINGLE example of erroneous information.

Wikipedia knows everything, duh.

You're not supposed to. Even Wikipedia tells you that.

Wikipedia can be used a source of keywords that you can use to search and find (Google) other authoritative sources that either verify or refute the info found on Wikipedia. It can give you an overview of the subject so that further research on the web yields proper definitive sources.

people contribute to it

In most academic institutions, (such as college), Wikipedia and personal web pages are considered unacceptable sources. When anyone can update the page and exact sources are unknown, it cannot be a reliable source. Please ask questions without swearing. It didn't add anything to your question.

well, you really shouldn't because it's not considered a proper reference. But I suppose you could cite it just like any other online reference.