How I believe it works(this isn't fact, it's just what I think) that sometimes after people die they get trapped here, for whatever reason. Ouiji boards SOMEHOW are able to communicate with the spirits in whatever building you're in, implying they're at the board themselves, we just can't see them. I don't really know why that is, and some days are active(spirits roam the building) but other days they vanish. And yes... Sometimes I've heard, using ouiji boards can summon bad energy... So maybe, there are demons, or maybe they're just vengeful spirits. Who knows.