A balloon -- any balloon, which could come from the audience -- is inflated, placed across the room, and popped at command. The sound and explosion effect make this a perfect followup for an effect that you'd rather not take into examination. Makes a nice clean finish for a previous effect, demonstrates mental psychokinetic skills, can be very effective in comedy act.
"If you are a psychic sensitive, you will actually be able to see my astral body float across the room in this demonstration. I'll be the dark, purplish cloud.
"What I'm going to do is to have someone choose any balloon out of this heap of balloons, or if you happened to bring a balloon with you, we can use that. I'm going to ask you to inflate the balloon and tie it at the nozzle. We'll then place the balloon here at this side of the stage.
"I'm going to go over to the other side of the stage. Please notice that at no time did I touch the balloon.
"I will now be tied to this chair and blindfolded. I will now enter into a hypnotic trance, leave my body and float to the other side of the stage, at which time I will do something dramatic to indicate my presence near the balloon." ... and then POP! the balloon explodes on YOUR command!