Many years ago, when I worked as a volunteer at Stanford Hospital, I got to know a little girl named Liz who was suffering from a rare and serious disease.
Her only chance of recovery appeared to be a blood transfusion from her 5-year old brother, who had miraculously survived the same disease and had developed the antibodies, needed to combat the illness.
The doctor explained the situation to her little brother, and asked the boy if he would be willing to give his blood to his sister. I saw him hesitate for only a moment before taking a deep breath and saying, "Yes, I'll do it if it will save Liz."
As the transfusion progressed, he lay in bed next to his sister. He looked up at the doctor and asked with a trembling voice, "Will I start to die right away?" Being young, the boy had misunderstood the doctor; he thought he was going to have to give his sister all of his blood.
(Note: I'm not sure where I originally got this one from... there was no mention of the author. If you know the author, please email me so I can give credit! Thanks)