A king sent 10 of his servants out to buy him each 10 gold rings. Each ring should weigh 10 grams, and altogether there should be 100 gold rings.
One of the King's spies told him that one servant had cheated and had brought 10 rings each of 9 grams instead of the desired 10 grams. Unfortunately, the spy hadn't been able to recognize the face of the servant, so that the king was left wondering which of the servants had stolen his money.
In order to discover the unfaithful servant, the king used a digital balance but only once.
How could he do that?