Add "steel sharpens steel" as another metaphor that I love to hate. Steel does not, in fact sharpen steel. Two swords clashing dull each other. And even when a steel refers to a diamond steel, a diamond steel only hones a blade, it doesn't sharpen it.
O.o Do you mean iron sharpens iron? As in the biblical quote? - Soup in a TARDIS
Technically, a sharpening stone not only doesn't sharpen but it doesn't even really truly hone: http://en.wikipedia.org/wiki... - Stephen Mack
But I suppose "undeforming steel" wouldn't catch on. - Stephen Mack