I actually tried the 2-4-6 puzzle on both my brothers, and they both got it right because they thought there was some trick to it and so kept pressing until they were sure (and even after ~20 questions still didn't fully endorse their answers). Maybe I have extra-non-biased brothers (not too likely), or maybe the clinical 2-4-6 test is so likely to be failed because students expect a puzzle and not a trick. That is to say, you are in a position of power over them and they trust you to give them something similar to what they've been given in the past. Also there's an opportunity cost to keep guessing in the classroom setting, both because you have less class time to learn and because if other students have already stopped you might alienate them by continuing. Point is, I've seen markedly better results when this puzzle is administered in a casual or 'real-world' setting. I intend to try it on other people (friends, co-workers), and see if the trend continues. Anyone else tried it and gotten this result?