Wikipedia:Tip of the day/April 15
Everything is copyrighted by default, even if it does not have a notice to that effect.
But most of Wikipedia's text and many of its images are co-licensed under the Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA) and the GNU Free Documentation License (GFDL).
That makes Wikipedia copyleft!
Wikipedia articles can be copied and modified by anyone, but there is a catch: All modifications have to be made available under the same conditions, and credit has to be given to the original authors.
However, when editing Wikipedia, please only add content which you have created, or which you have been permitted to use under the terms of the licenses mentioned above. Otherwise, it will be removed.