What is the mean of the four numbers 2, 4, 6 and 8?

Explication

The four numbers total 2 + 4 + 6 + 8 = 20, and 20 divided by 4 values gives a mean of 5. Answering 4 comes from dividing the total by 5 instead of by 4, which counts one more value than the list actually contains and so shares the total too thinly.