Plagiarism (taking someone else's work or idea and offering it as one's own) is academic dishonesty. Examples include copying someone else's work word-for-word, paraphrasing, and borrowing ideas and putting them in different words. Cutting and pasting material found on the internet is another common example of plagiarism. ---'Iolani School Catalogue.
Common knowledge is information that is accepted and known widely you do not need to cite it:
Facts that can be easily verified.
As you are conducting your research on a topic, you will see the same
facts repeated over and over. Example: You are writing a paper on
nuclear power . The heat and light
of the sun result from nuclear energy. Although you might not have known
this fact before your research, you have seen it multiple times and no
one ever argues about it.
Facts that you can safely assume your readers know. Examples: Honolulu is the capital of Hawaii. Fish breathe using gills. The earth is composed of tectonic plates.
Common sayings or cliches. Examples: Curiosity killed the cat. Ignorance is bliss.
Not all facts are common knowledge. You will still need to cite:
Facts that surprise you or your reader. Examples: The theory of plate tectonics gained widespread acceptance only in the late 1960s to early 1970s. (Lerner 3139)
Facts that include statistics or other numbers. Example: The two largest recorded earthquakes were the magnitude 9.5 Chilean earthquake of 1956 and the magnitude 9.2 Prince William Sound, Alaska, earthquake of 1964 (Hanneberg 1324)
If you use the exact words of another writer, even if the content within could be considered common knowledge. Example: The United States Geological Survey estimates that more than three million earthquakes occur on Earth each year. (Hanneberg 1323)
Common knowledge can be course-specific. For example, the number of bones in the leg could be considered common knowledge in biology course. But if you are using that fact in an English paper, you cannot assume your teacher would have that knowledge, and you would need to cite it.