Despite huge advances in sleep science and our understanding of the human brain, our understanding of what on earth specifically makes us shut our eyes and vividly hallucinate for 8 hours a night remains pretty mysterious. It's been suggested that we evolved dreaming as a way of consolidating memories, helping us solve problems we couldn't deal with consciously, or performing necessary brain repair functions. It remains, however, one of the oddest parts of universal human experience. While newborns look as if they're dreaming, it seems that dreaming actually develops into something sophisticated in early childhood, requiring advanced brain connections that don't really exist until we're perhaps 4 or 5. The fact that even in the age of MRIs dreams remain pretty opaque indicates just how complex they are; so it's remarkable to see the insights and beliefs assigned to dreaming by ancient civilizations. Some of them, needless to say, are extremely odd.
The idea that dreams could predict the future is one of the most ancient ideas around; the practice of "incubation" — of staying in a certain sacred place and doing rituals to welcome prophetic or helpful dreams — has been found across various ancient cultures. However, there's a lot more to dreams and their history in civilization than that. From spells to demons, livers to pregnancy, here are some of the oddest beliefs about dreams; your grandmother's insistence that bad dreams come from indigestion looks basically sane by comparison.