You have two parts of [your] brain, "left" and "right". In the left side, there's nothing right. In the right side, there's nothing left.
Left Brain, Right Brain
Explanation
Explain Bear
Listen up, genius. This joke is all about how the words "left" and "right" can also mean "correct." The joke says that the left side of the brain ain't correct, and the right side ain't got anything remaining. Get it now, smart guy?