How to avoid plagiarism

Everybody is worried about plagiarism, but how do you define what is plagiarism. A lot of institutes have been forced to make some definitions, commonly just using examples. General agreement is that plain copying of existing material is definitely plagiarism. Detecting the copying is not always easy. Today, there exist tools that make the detection quite automatic. Earlier the detection was based purely on the teacher knowledge about the subject. Later, Google and similar search engines made it possible to search for certain phrases by hand. Now the tools do the matching automatically. However, the detection is just one part of the story. It is much more important that the authors do not copy from others, but say it using their own voice and words.