Question: You have 7 tennis balls that are all identical but one of them is slightly lighter than the others.
Using a balance scale and only two separate weightings, how can you find the light tennis ball?
Answer: Put three of the balls on each side. If they are even the ball that wasn't weighed is the light one. If they aren't even the side that is lighter has the light ball. Of these three balls, one should be put on each side. If the sides are even than the other ball is the light one. If they aren't even the one that is lighter is the ball you're looking for.