Ancient Japanese swordsmiths knew that to make a razor-sharp sword, one needed hard steel. But, hard steel was brittle - it was not flexible and would break easily when struck against another weapon. Soft steel, on the other hand, was very resistant to breaking, but was too soft to hone to a sharp cutting edge.
Thus, the swordsmiths created the method of repeatedly hammering together a layer of soft and hard steel, folding it over, heating it - then hammering, folding, heating... Many many times the metal was subjected to extreme force and heat.
Eventually, a blade began to form, a merging of the weak-but-flexible and the hard-but-brittle creating a blade that could withstand vast pressure, yet remain so sharp as to slice through the toughest muscle and bone dozens of times over: The katana was born.
This analogy is used sometimes to illustrate the properties of yin and yang - merging two opposite ideals to create a balanced and powerful whole. Othertimes, to me at least, it also illustrates how the strongest people are often those that have been forged harshly in life - hammered, bent and folded, gone through the fire and come back. Experiences that were so sharp and emotional they could be used to cut at others, yet so brittle the heart easily shatters; the times in life that were so peaceful and pleasent they make you feel as if you could deal with anything, but would fail to defend in times of need - these two, weak on their own, can be forged to create a perfect blade, a perfect being.
The question I often ask myself is - what or whom wields the hammer, and what is my anvil?