Zeng Zi, a student of Confucius, was faced with a moral dilemma when his wife went shopping. To quiet their crying child, she promised to slaughter their pig for a feast upon her return.
When she came back, Zeng Zi proceeded to capture the pig for butchering. His wife stopped him, saying, "I was only joking with the child." Zeng Zi replied, "One should not joke with children. They know little and learn by imitating their parents and following their instructions. If you deceive them, you are teaching them to deceive others." Consequently, Zeng Zi killed the pig.
He understood that sincerity and keeping one's word are fundamental principles of personal conduct. Breaking his promise might have saved the pig, but it would have left an indelible shadow on his child's innocent heart.