"Answer the following questions as fast as you can:
-2 + 2 = ?
-4 + 4 = ?
-8 + 8 = ?
-16 + 16 = ?
Now quick! Pick a number between 12 and 5. Got it?
The number you picked is 7, isn't it?"
...I succumbed to this piece of 'mindreading' when I read it in Stanislas Dehaene's 1997 "The Number Sense: How the Mind Creates Mathematics." Did you?
He calls this a "demonstration of the automaticity of arithmetic memory" and explains it thusly:
"How did I read your mind? The mere presentation of the numbers 12 and 5 seems enough to trigger an unconscious subtraction 12 - 5 = 7. This effect is probably amplified by the initial addition drill, the reversed order of the numbers 12 and 5, and the ambiguous phrase 'between 12 and 5' that may incite you to compute the distance between the two numbers. All these factors conspire to enhance the automatic activation of 12 - 5 up to a point where the result enters consciousness. And you believed that you were exercising your 'free will' when selecting a digit!"
I'm not sure I find Dehaene's explanation completely satisfactory... but, I can't argue with the effect, which I did fall for.