There are twelve ball bearings, all the same size, shape and colour. All weigh exactly the same, except that one ball bearing is slightly different in weight, but not noticeably so in the hand. Moreover, the odd ball bearing might be lighter or heavier than the others.
Your challenge is to discover the odd ball bearing and whether it is lighter or heavier. You must use a beam balance only, and you are restricted to three weighing operations.
A beam balance is a simple scales that has two trays, one on either side. Whichever tray contains the heavier load will go down, the other tray up. So it only tells you which side is heavier, but doesn't tell you the actual weight of either side.