Why is his mouth sewn ?
We read, that Loki lost a bet once with two dwarfes and the stake was his head, but when it came to cut it off, he said, that his neck was no part of the deal. The dwarf Brokkr got so angry, that he sew his lips together to teach Loki a lesson (didn´t help by the way :))
the whole poem is very complex and explains a lot of things, f.ex. where the Thor´s Hammer is coming from, why Sif´s hair is golden, where Odin got his spear and ring from etc., etc.
(old norse original text with english translation)