The Mistaken Laugh
Parshat Vayeira tells about an angel - disguised as an itinerant traveler - coming to Abraham and Sarah to inform them that they'll finally have their first child together. This was a big surprise, given that Abraham was 99 years old, and Sarah was 90!
In response to this startling news, Sarah laughed in disbelief.
Nachmanides says that it was a mistake for Sarah to laugh. Rather, she should have answered "Amen" - "so shall it be" - because when somebody gives us a blessing, we should never treat it lightly.
We learn this principle from an incident that occurred to Rabbi Yishmael, the high priest in the Temple who entered the Holy of Holies on Yom Kippur. It was there that God Himself asked Rabbi Yishmael to give Him a blessing! (By the way, Rabbi Yishmael blessed that God's mercy should overcome His strictness.) Yet we see from here the idea of not taking anyone's blessing lightly.
So what is the proper response when someone gives us a blessing for good health, or for success with one's family? We must believe in the power of every human being to be a conduit for God's blessing. So we graciously say, "Amen." And we add: "So shall it be for you, too."