Common knowledge does not need to be cited. Common knowledge includes facts that are known by a lot of people and can be found in many sources. For example, you do not need to cite the following:
- In 1865, following the assassination of Abraham Lincoln, Andrew Johnson was elected as the 17th President of the United States.
- William Shakespeare was born in 1564 and he died in 1616.
- A genome is all the DNA in an organism, including its genes.