You are given 12 seemingly identical metal balls, but one of them weighs slightly more or less than the others. With a balance scale and three weighings only, how can it be determined which is the oddball and whether it's heavier or lighter?
This is one those puzzles that'll likely take a fair amount of time to work out, but the solution (at least the only one I know of) is ingenious and very satisfying to discover. No special math or logic skills are required, just hard work, perseverance and insight.