Look at the following question:
Which of the following doesn't belong?
Did you pick "penny" because it's the only one that's not silver? According to one IQ test by Hans Eysenck (I have it in dead-tree form, so I can't link to it), you'd be correct. But this irks me: what if you happened to notice that the dime is the only coin with an even-numbered value (25¢, 10¢, 1¢, 5¢)?
I'm curious as to how people create IQ tests. What makes people decide, for questions like the one above, that this unique trait is correct while that unique trait is beneath consideration? Is there something more "obvious" about the respective colors versus the respective values of each coin? I admit I don't get it... which is probably why I'm not a genius.