A child who throws a doctor's hands

The boy lives with his sister. Her parents died early. She was his only relative. So boys love their sisters better than themselves. However, the disaster once again came to these two unfortunate children. My sister is seriously ill and needs blood transfusion. But the hospital's blood is too expensive, and the boy has no money to pay for it. Although the hospital has been free of the surgical fee, the sister will still die without blood transfusion.
As the only relative of his sister, the boy's blood type is consistent with that of his sister. The doctor asked the boy if he was brave and had the courage to suffer from the pain. The boy began to hesitate. After thinking, the 10-year-old brain finally nodded.
When the blood was drawn, the boy quietly smiled at his sister in the neighboring bed. After the blood was drawn, the boy trembling and asked, "How long can I live, doctor ?"
The doctor was trying to laugh at the boy's ignorance, but he was shocked by the boy's courage: In the boy's 10-year-old brain, he thought that blood transfusion would lose his life, but he still gives blood to his sister. At that moment, the boy made the decision to be brave and make up his mind to die.
The doctor's hands sweated. He clasped the boy's hand and said, "Don't worry, you won't die. Blood Transfusion will not lose life ."
The boy's eyes shine: "really? How many years can I live ?"
The doctor smiled and said, "You can live to 100 years old. You are healthy, young man !" The boy was so happy that he jumped again and again. When he confirmed that he was really okay, he rolled up his arm again -- the blood-drawn arm raised his head and solemnly said to the doctor, "That's the best thing to do, each of us lives for 50 years!"
Everyone is shocked. This is not a child's unintentional promise. It is the most selfless and pure promise of mankind.