Give an example of a negative correlation and explain why it is not causation.

A negative correlation is one in which as one variable increases, the other goes down. Some studies have shown that as the use of video games increases, the gpa score decreases. That's a negative correlation. 

A negative correlation can never be a cause. The reason is the direction each of the variables is going. If the thing that might cause the other variable to go down, then the first variable is not a cause.

You need a positive correlation to claim that one is the cause of the other. 
So for example, if you contribute to and answer questions, and give give answers, your understanding of the subject matter is likely to go up. 

Giving good answers could be a cause for better marks.