Well, I don't want to spoil all the fun, but here is a story about the son of someone my mother used to know.
For his 18th birthday he invited some friends for a weekend in a castle. One night they decided to call the spirits of death people. That boy was the medium through which the spirits could talk to them. It all started as a joke, but ended in tragedy. They didn't expect anyone to answer, until suddenly the boy started to speak with the voice of his dead grandfather. He completely freaked out, jumped out of the window in madness and by this killed himself.
So, be careful when trying out things like this.
It's not because we can't prove it (yet) that is isn't true.
(the truth is out there! ;) )