The magician writes in a chalkboard a prediction and asks any audience member to name any number, letter, word or anything he wants to predict. The magician turns the chalkboard and the prediction matches with the freely thoughts of the audience.
Put the blackboard without anything written on it and a chalk inside an envelope and leave it on a table. Now, ask ghosts to show they are present by writing on the blackboard. Then, you will take it out of the envelope and read the messages from the other world!