I Can Read Your Mind!
Pick a card, any card...
Here is a well-traveled card trick / mindreading illusion that keeps popping up on the Internet, most recently in the form of a PowerPoint presentation purporting to be (though it almost certainly is not) the work of master stage magician David Copperfield.
The illusion can be startling -- until you figure out how it works, at which point you may find yourself wondering how anyone could possibly fall for such a simple, obvious deception.
It goes something like this...
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