Question

When a writer uses “common knowledge” in an essay, why does it NOT require a citation nor is it considered plagiarism?

Answer 1

Answer:

Common knowledge is a kind of knowledge which everybody, or almost everybody, knows. Because is not written in any place cannot be cited, nevertheless, it is accepted. It is not considered plagiarism because it belongs to a big part of the population, as a consequence, it doesn't have an author.

Answer 2

Plagiarism is when you just copy exactly what someone else says, without their permission, or without citing where you got it from. 
Common knowledge should be your own words. unless you take it from somewhere else, it doesnt need anything.