The same sort of thing you might hear watching Kung Fu.
Body builders know that as they lift weights, it creates small tears in the muscle that are replaced with other muscle cells.
The principle holds through a lot of things in life. A show on the Discovery Channel or one of it's affiliates that we saw called Xtreme Martial Arts tells how the process of breaking boards causes small cracks in the bone being used. The healing process of the body, like with muscles, repairs these cracks, filling them in, making them harder.
I think that many of us are so comfortable that we are afraid of challenges. In fact, we are more than willing to whine when things get tough. We forget that they do build us up.
Remember what James 1:2-4 says:
Consider it a sheer gift, friends, when tests and challenges come at you from all sides. You know that under pressure, your faith-life is forced into the open and shows its true colors. So don't try to get out of anything prematurely. Let it do its work so you become mature and well-developed, not deficient in any way.