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An introduction to Creative Commons
The key thing that will enable or prevent knowledge sharing is compatible content licensing. E.g. if you and another project are both on a Creative Commons Attribution ShareAlike license, (the license used by Appropedia), you can share content in both directions, which is perfect. This license allows for commercial use, avoiding any uncertainty over use with advertisements, and is compatible with various other sites & /projects as well. Someone could adapt an Appropedia page, adding their own ideas, and then those additions could be used on Appropedia or elsewhere - with attribution back to the party that made the additions.
For complete beginners
Those links don't try tell help you decide which content license is right for you.
Part of the reason we use CC-BY-SA rather than using the NC (NonCommercial) clause is found at Non-commercial licenses vs open licenses. The key points are that if you use a NonCommercial license, we are restricted from using your work in Appropedia, and that the risks of commercial exploitation are probably much less serious than you imagined, especially if you use the ShareAlike clause.
A much more extended argument is found at The Case for Free Use: Reasons Not to Use a Creative Commons -NC License.
If you and the people you want to collaborate with are happy with the same license, then you can talk with your potential partners about details, and technical issues, such as how to ease the process of sharing between the sites.
Public domain vs Creative Commons
If you do choose public domain, you can use a "public domain mark" from Creative Commons, through their normal license mark system (see below) (or elsewhere?[verification needed]) to let search engines find and index it.please expand
How to add a Creative Commons mark
If you have decided on a Creative Commons license and are ready to add it to your site, see Marking your work as Creative Commons.