When Loki had Sif's hair, Freyr's ship Skidbladnir and Odin's spear Gungni done by the sons of Ivaldi, he bet his own head with Brokk that his brother Eitri wouldn't have been able to make items worth as much as the ones above.
Eitri began working in his furnace while his brother was working the bellows, but a fly (Loki himself?) began stinging Brokk, trying to stop him and spoil the items.
Eitri succeeded in making a golden boar, Gullinbursti, a golden ring, Draupnir and the hammer Mjollnir that made his brother win the bet, even if its handle was shorter than it should have been.