A staple for attaching together first and second metal sheets is made of a length of round wire bent at the ends to form spaced apart depending metal legs, each having a metal piercing point at the end thereof adapted to pierce completely through the first sheet and at least partially through the second sheet and also adapted to spread the legs further apart during the piercing to a spread position in which the legs are anchored within the metal sheets and thereby attach the metal sheets together. The metal piercing points each having an outside bevel surface on the outside of the leg, an inside bevel surface on the inside of the leg, and a blunt end surface between the inside and outside bevel surfaces.