"The Japanese monkey, Macaca fuscata, had been observed in the wild for a period of over 30 years. In 1952, on the Island of Koshima, scientists were providing monkeys with sweet potatoes dropped in the sand. The monkeys liked the taste of the raw sweet potatoes, but found the dirt unpleasant. An 18 month old female named Imo found she could solve the problem by washing the potatoes in the salty ocean water, improving the taste of the potato. She taught this trick to her mother. Her playmates learned this trick and taught their mothers too. This cultural innovation was gradually picked up by numerous monkeys in the troop and observed by the scientists.
Between 1952 and 1958, all the young monkeys learned to wash the sandy sweet potatoes and make them more palatable. Only the adults who imitated their children learned this cultural improvement. Other adult monkeys kept eating the dirty sweet potatoes. In autumn of 1958, something startling took place. A certain number of Koshima monkeys were already washing their sweet potatoes, the exact number is not known. The hypothetical number given was 99. Then it happened. The hundredth monkey learned to wash the sweet potatoes. The added energy of that hundredth monkey somehow created an ideological breakthrough. Almost everyone in the tribe was washing their potatoes before eating them, but a surprising occurrence was observed by these scientists. The habit of washing the sweet potato had jumped overseas. Colonies of monkeys on other islands and the mainland troop at Takaskiyama began washing their sweet potatoes.
Although the exact number may vary, this Hundredth Monkey Phenomenon means that when only a limited number of individuals knows a 'new way', it remains the conscious property of those individuals. However, when one more individual manifests this new awareness, the field is strengthened, a critical mass is reached, and the awareness becomes the conscious property of all. This new awareness is communicated mind to mind."

From The Hundredth Monkey by Ken Keyes Jr.