Could you be so brave?

In the news lately ,there have two stories of heroes

One was about a man in New York who saw a young man fall onto the subway tracks. he jumped down to try to drag the young man to safety. He thought he would not have time,so He threw the man down and lay on top of him in a small area on the tracks. Some of the subway cars rolled over him. They both survived because of his bravery. I wonder if I have that in me to save a persons life Could I be so brave in this situation?
What about the young soldier who covered a grenade with himself to save his fellow soldiers. We hear some stories of wrong doing in the war by soldiers but there are so many more brave men and women giving their lives to defend this country.
I just hope that if I am faced with a situation I will be brave enough to do what is needed.