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The phrase at the beginning of a number of silly number games and tricks, which usually are supposed to prove something about the mind-reading powers of the demonstrator, often a magician or math teacher. Of course, they usually have more to do with illusion, basic math, or coincidence.

For example:

Think of a number. Double it and add 2. Triple the result and add 3. Subtract the original number. Subtract 4 from the result. Subtract 5 and divide by 5. Wow, you're back at the original number! :P

Or, for slightly more advanced minds:

Think of a three-digit number in which each succeeding digit is smaller than its predecessor (752 for example). Reverse that number (257). Now subtract the smaller from the larger (752-257=495). Reverse that number (594). Now add the two together (495+594). No matter what your initial three-digit number was, you'll end up with 1,089. Wonders never cease.