Which would be your biggest act of bravery to defend your friends?

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I was watching TV this week and I saw on a TV show that a man jumped in the rail of the subway to save a kid that had fallen. I thought that was such a big act of courage, specially because he didn't even knew who the kid was. I probably would not have the same courage that this hero even if were some of my friends or family. I was wondering, did you ever made an act of bravery to defend your friends or your family? If no, what do you think that you would be capable of do to save someone you love?