A baker and a farmer had an agreement, the baker got butter from the farmer and the farmer got bread from the baker. After a while the baker noticed that the pieces of butter from the farmer, that should weigh three pounds, became lighter and his scales agreed with him. He got angry and went to court to complained about his butter supplier. “The baker claims your pieces of butter do not have the required weight” the judge said to the farmer, “this piece of butter should weigh three pounds, but it weighs much less.
“That's impossible, Mr. Judge” the the farmer said, “I check the weight every time.” The judge than said: “Maybe your weights are incorrect!”. The farmer replied in dumb amazement: “My weights? I do not have any weights, I never use weights.” The judge replied: “So if you do not have weights, how do you check the weight of the butter?”. The farmer said: “Quite simply, I get my bread from the baker and he gets butter from me. A loaf of bread weighs three pounds so put my butter on the left scale and a bread on the right side!”