Q: Why do flamingos stand on one leg?
A: If they lifted the other leg they would fall over!

Explanation:  Not what you expected?  Of course not!  That’s why this joke is funny.

Upon asking the question one expects an answer, an explanation.  In this case one expects an answer that tells why flamingos stand on one leg.  This joke is funny because it focuses on the other leg… and that is unexpected.

¡Saludos a Cesar!