Free software is different from free music or text, because it is often much more difficult to understand and change a program if one does not have its source code, than it is to change a song or a text. Because of that, the source code of free software must always be easy to receive, or the software will not be free. However, source code for pictures, movies and music can also exist and be given to others. Choosing what to call a “source code” is sometimes important when publishing a work under a copyleft license.