A classic logic puzzle.
On the table sit nine identical looking nuggets of gold. However, you know that one of the nine is a clever forgery. The only difference between the fake and the real nuggets is in weight: the fake gold weighs slightly less than the true gold. Unfortunately, this weight difference is not great enough to be noticed in human hands. Fortunately, you have a standard balance scale. Unfortunately, you may only use the balance scale twice. Fortunately, there is a way to find the fake gold even with these restrictions. How?
Twelve weights - you can use the scale three times to find a weight that is either lighter or heavier. Quite a challenge.
Six weights - you can use the scale twice to locate the heavy weight in each of three colors.