Long ago, there’s happens to be a boy. Every day, he used to go to the school. One day he was very hungry while returning home. Further continuing to walk he became hungrier. When he reached home, he found no one and also noting to eat as his mother has gone for work. Then he thought of going to his neighbor to ask someone for some food. There was a girl living nearby his house. That pretty girl offered him a glass of milk. He certainly drank it.
The boy offered her payment for the milk but out of generosity girl refuses to take. Many years have passed. Girl was now grown up and had some serious illness.
She had checkup with many qualified doctors but those doctors couldn’t cure her illness. After some while she got to meet with a best doctor in town and went there for checkup. In couple of months she was cured.
She felt very excited that finally her illness has gone. On the other hand, she thought she couldn’t pay the doctor’s fee. In no time hospital gave her a not instead of bill. There was written, your bill has been already paid with “a glass of milk”.