"What Good Is A Glass Dagger?" is a fantasy novelette written by Larry Niven and published for the first time in 1972 in The Magazine of Fantasy & Science Fiction.[1] The story occurs in the same fictional universe as the book The Magic Goes Away, by Niven, being the second story in this universe, the sequel to the short story "Not Long Before the End".[2] Asimov comments that in this story "Niven tries to give a scientific explanation for magic."[3] The concept of mana appears in Niven's fictional universe, and from it has spread to the RPGs. However, it is not original to the series, as Niven said he learned about it by reading the book The Trumpet Shall Sound (1968).[4][5] Niven also invokes the principle of conservation of mass at some point in the narrative.[6]