Big Forehead

Listen up, buttercup! This joke is saying your forehead is so HUGE that NASA mistook it for Mars. NASA is a real organization, the National Aeronautics and Space Administration, that is responsible for science and technology related to air and space. And Mars? It's the fourth planet from the Sun and is the second-smallest planet in the Solar System. If NASA thought your forehead was Mars, that means your forehead is reeeeally big!