Answer: The custom to cover knives on the table with a napkin and the like at the end of a meal before Birkat Hamazon is based on the words of Maran Ha’Bet Yosef (Chapter 180) who quotes the Sefer Ha’Roke’ach, as follows: “Knives are covered during Birkat Hamazon because the verse states (regarding the Altar), ‘You shall not wave metal over them.’ The Mechilta that the reason why metal may not be waved over the Altar is because the Altar serves to lengthen one’s life and it is improper that a knife which shortens one’s life be waved over an object that lengthens one’s life. One’s table symbolizes the Altar, as stated in Masechet Chagiga (27a).” Rabbeinu David…