:Hello, Ansegam. I'm afraid that it is a common misunderstanding that because something is found on the internet, it is public domain. This is not true - under the U.S. law that governs us, all content is copyrighted immediately upon creation and remains that way until copyright expires (usually 70 years after the author dies) unless it is ineligible. We can use content that is ineligible, like content that is published by the U.S. government (although not all government content is ineligible) or that has been explicitly licensed or released for use by its authors. The vast quantity of what you find on the internet does not count. 

 :Being free of charge is a very different thing from being free of copyright! For instance, Wikipedia itself is free of charge, but it is not free of copyright. People are allowed to copy things from Wikipedia, but only if they meet the terms of the license.