The oldest one in the book.
You get your friends together and decide to sit down in a dark room and try to contact spirits. You get out the Ouija board, and everyone simply places their hands on an indicator, called a planchette, that moves around a board and points to letters in order to spell out words or phrases. Before you know it, your long-dead grandma's back in touch and you're having to explain your masturbation habits to her.
"It's kind of an up-down stroking motion, really. Why are you even asking me this?"
What's awesome about Ouija boards is that the real reason it works is almost as spooky as the ghost explanation: You're actually communicating, not with the dead, but with the subconscious part of your brain.
"M...I...L...K...B...R...E...A...D... guys, I think my subconscious is really fucking boring."
Your hands move the piece across the Ouija board due to involuntary movements in your muscles, which are called the ideomotor effect. Basically, your brain can and will move your muscles without your express permission because, for the most part, your body kinda operates on autopilot anyway. It's just usually not brought to your attention (you'll notice it the next time a light stops working in a room, but you unconsciously keep flipping the switch every time you walk in anyway).
So with the Ouija board, you subconsciously think of a response to the question and your brain subtly moves the planchette where it wants it. Maybe not enough for it to work if you were using the board alone (though it is for some people -- it's likely how water dowsing works), but when you get a few people together and they're all subconsciously pulling, it creates the distinct sensation that the planchette is moving on its own accord.
"The Cosmic Vibrations demand that you go down on me."
It's so weird that the explanation itself sounds like bullshit. But if you want further proof that it's us humans doing all the soothsaying, just check out this experiment, where magicians Penn & Teller blindfolded some random people, flipped the Ouija board 180 degrees and had them try to contact the spirit of the guy who played Fred Mertz on I Love Lucy. The results are less than startling.
In theory, ghosts should be able to direct their hands no matter the orientation of the board, right? Turns out, without being able to see the board, they just kinda move their hands to where they think the letters are.
3The Psi Wheel
This trick begins with someone, probably a douche, claiming he can move stuff "with my mind, baby." If this person doesn't put his hand on your knee next, then he's likely going to pull out three things: some kind of base, usually an eraser; a pin, which he sticks in the eraser; and a small piece of paper or tinfoil folded into a sort of umbrella-looking shape.
Pictured: Tiny people relaxing at Eraser Beach.
The would-be mystic puts his hands around the contraption and focuses really hard. Lo and behold, the paper umbrella begins to spin without anyone touching it. He really does have magical powers! He puts his hand on your knee.
Check out this kickass demonstration:
Simple convection is what is "psychically" moving the wheel. Simple convection and hubris, but mostly convection. And convection, in case you didn't retain any science class at all, is that thing where heat moves molecules in a gas or a liquid. It's why water gets bubbly when it boils and how hot air balloons rise and why you've really got to be careful when handling a hot crack pipe. Here's what convection looks like:
So in the case of a psi wheel, the mystic cups his hands around the base of the gadget and heat coming off his hands rises, which draws in cool air, creating a current that turns the paper. The same effect can be achieved by placing hot cups of any liquid around the setup: The heat rises around the paper, cool air rushes into the gaps between the cups and magic time begins.
Or these cups are just full of magic.
Believe it or not, one guy made something of a living out of convincing audiences he could move tiny objects with his mind. James Hydrick didn't even bother with the convection thing, either. He just blew on shit. Here's the national appearance that killed his career. Would it shock you to know that a guy dressed like this is now a convicted sex offender? Didn't think so.
Step away from the phone book, Hydrick.