Ouija Boards and Dowsing Rods

Why do they move?  Because we make them move (h/t nc links):

Ouija board cups and dowsing wands – just two examples of mystical items that seem to move of their own accord, when they are really being moved by the people holding them. The only mystery is not one of a connection to the spirit world, but of why we can make movements and yet not realise that we’re making them.
The phenomenon is called the ideomotor effect and you can witness it yourself if you hang a small weight like a button or a ring from a string (ideally more than a foot long). Hold the end of the string with your arm out in front of you, so the weight hangs down freely. Try to hold your arm completely still. The weight will start to swing clockwise or anticlockwise in small circles. Do not start this motion yourself. Instead, just ask yourself a question – any question – and say that the weight will swing clockwise to answer “Yes” and anticlockwise for “No”. Hold this thought in mind, and soon, even though you are trying not to make any motion, the weight will start to swing in answer to your question.
Magic? Only the ordinary everyday magic of consciousness. There’s no supernatural force at work, just tiny movements you are making without realising. The string allows these movements to be exaggerated, the inertia of the weight allows them to be conserved and built on until they form a regular swinging motion. The effect is known as Chevreul’s Pendulum, after the 19th Century French scientist who investigated it.
What is happening with Chevreul’s Pendulum is that you are witnessing a movement (of the weight) without “owning” that movement as being caused by you. The same basic phenomenon underlies dowsing – where small movements of the hands cause the dowsing wand to swing wildly – or the Ouija board, where multiple people hold a cup and it seems to move of its own accord to answer questions by spelling out letters. The effect also underlies the sad case of “facilitated communication“, a fad whereby carers believed they could help severely disabled children communicate by guiding their fingers around a keyboard. Research showed that the carers – completely innocently – were typing the messages themselves, rather than interpreting movements from their charges.

I've tried using dowsing rods to find tiles, and it never fails that the rods tell me the tiles are in the low areas where I thought they'd be.  But when I dug for them....not so much.  Now, I know a lot of people who know somebody who is fabulous at dowsing, or at least that's what they say.  I'll go with the unconscious mind explanation.