My grade 12 math teacher gave us a question that I found to be very interesting. You have 12 identical keys. 11 of the 12 keys have the same mass. Only one of them has a different mass. You have a scale, but it is not digital and it does not tell you the actual weight. It only shows you which one of the two sides is heavier. You are allowed 3 uses of the scale. How do you determine which key has a different mass? I found this question to be very interesting, but unfortunately, I can't solve it. How about you guys?