The two smiths had a contest to prove which one would be the greatest sword in Japan. The two swords, the Masamune and the Murasame were thrown into the river. The Murasame drew all the nearby leaves into it, cutting it to pieces. However, the Masamune repelled all of the leaves near it. Thus, the masamune was declared Japan's greatest sword.
I didn't make this up. i looked it up and as a swordsman I thought I should retell the story.