Content in Copyleft, Public Domain, Creative Commons. Check license before use.
Even in the case of Public Domain, we recommend taking the Copyleft approach:
From wikipedia:
The underlying principle is that one benefits freely from the work of others, but any modifications one makes public must be released under similar terms. For this reason some copyleft licenses are also known as reciprocal licenses, they have also been described as "viral" due to their self-perpetuating terms.
Some Creative Commons licenses maybe more restrictive, such as non commercial (cc-nc)