Nasruddin catches hare; he has never seen one. He tells wife not to touch the bag; he is going to go get doctor to ask what it is. Wife is curious; opens bag, out jumps hare, runs off. So she puts metal measuring cup in the bag. Nasruddin brings doctor and judge and other friends to see. Nasruddin unties sack, dumps it out. Then Nasruddin says, "Eight of those make a gallon!"
Notes: Very similar to the story about the sack of gold when his wife replaces the gold with stones.