Because the dwarf Brokk with his brothers forged Frey's Skidbladnir, Odin's Gungnir and Sif's goldhair Loki made a bet with Brokk that he wouldn't be able to forge that kind of treasures again. Loki said he offered his head. Even though Loki created a fly that stuck Brokk in his head again and again he succeeded with his mission. He forged a goldpig, Gullinbursti, a goldring, Draupnir and a hammer, Mjollnir. Then they went over to Asgard to get the bet judged by Odin, Thor and Frey. The gods decided that Brokk had won the bet. The dwarf who didn't really cared for jewellery himself gave the treasures to the judges. Loki tries to escape, but Thor caught him. Brokk who felt sorry for Loki decided that he would only sew his lips together. It took a long time before Loki managed to rip up the seams.
Related Links:
Brokk Draupnir Gullinbursti Loki Mjollnir Thor
Aegir's feast
