The popular myth of how Thor killed the jötunn Geirröd has many variations, but all of them are caused by the trickster god Loki. Donning a suit of falcon feathers, Loki paid a visit to the jötunn's castle. When Geirröd saw the falcon, he knew right away that it was not a real falcon. Locked in a cage and starving, Loki revealed his identity. Geirröd released him on the condition that he bring Thor without his hammer to his castle. Loki readily agreed.