Why Documentation Matters
There are three main reasons documentation (citing your sources) is important:
1. It shows the author did background research and is knowledgeable on the
2. It creates a useful list of sources on a topic that helps other researchers.
3. It prevents the author from being accused of plagiarism.
Common knowledge - Some information--even if you had to look it up--is considered common knowledge and does not have to be documented. Examples are the distance to the Sun, Shakespeare's birthday, the population of the United States.
But be careful. Something you might consider common knowledge others, including your teacher, might not. It can't hurt to ask.