Cock egg

A cock egg, in archaic English, is a yolkless egg. Since they contain no yolk and therefore cannot hatch, these eggs were traditionally believed to be laid by roosters. This gave rise to the myth that when a cock's egg was hatched, it would produce a cockatrice, a fearsome serpent which could kill with its evil stare. According to the superstition, this could be prevented by throwing the egg over the family dwelling so it smashed at the other side without touching the roof.