Snorri Sturluson's Prose Edda describes Bifrost as a bridge built with great skill and craft, stronger and more cunningly made than any other structure, and identifies its familiar rainbow bands with fire: the reddish color visible in it, Snorri explains, is burning flame, placed there deliberately so that frost giants and mountain giants attempting to climb into Asgard would be turned back at the crossing.
The Poetic Edda, in the poem Grimnismal, has Odin describe the same bridge burning with flame and name it among the sacred places the gods ride to each day, since Asgard and the well of Urd beneath the world tree Yggdrasil, where the gods hold their daily council, sit on opposite sides of it. Only the gods and, in some readings, particular favored dead are said to cross it freely; Thor is noted as an exception who wades through the rivers below rather than risk the bridge's heat on his approach, which some sources connect to his elemental nature as a thunder and storm god rather than to any weakness in the structure itself.