What is the bravest thing you have ever seen someone do?

My friend donating (part of) her liver for her friend who had recovered from cancer.

It all sounded simple in the beginning because of the feats of modern medicine, but seeing what my friend went through (hospital stays, open surgery, recovery journey, risk to her own health...) that is a lot of sacrifice. And one of the bravest real thing someone does for another human being.