Once when Kyuzo Mifune visited a karate dojo, he was shown a demonstration of tile-breaking by one of the karate men. After the karate man had smashed a number of tiles piled on top of each other, he asked Mifune, "Can a Judo man do this?" "Yes, it is very easy," Mifune replied. "Is that so? Can we see what kind of technique a Judo man uses?" the karate man challenged. "Of course. Please set up the tiles. I'll be back in a minute," Mifune instructed. Mifune returned with a hammer he had brought along in his bag. "You are not going to use that to break the tiles, are you?" the karate man protested. "Yes. I told you it was easy. Efficient use of energy is a key principle of Judo."
Jigoro Kano in a 1936 conversation with Gunji Koizumi (Budokwai Bulletin, April 1947)