A man left for a business trip. As he had long suspected his wife in being unfaithful to him, he placed under their conjugal bed a jar with sour cream and hung a spoon above the jar. If one person rested on the bed, the spoon would not touch the cream, but with two bodies in bed, the spoon would be marked with cream. Having checked the device and confident that it worked reliably, the man left for his trip. In a week he came home, and his wife joyfully kissed and embraced him. "Just wait," the man thought. "I'll now find out how you behaved." He fetched the jar from under the bed and found the jar was full of butter.