The Japanese Macaca monkey has been studied in the wild for over 50 years. In 1952, on one small Japanese island, scientists dropped some 1 in the dirt for the monkeys. The monkeys liked their taste, but they found the dirt 2 .
One clever 18-month-old monkey found she could 3 the problem by washing the sweet potatoes in a nearby river. She taught this to her mother. Her 4 also learned this new way and they taught their mothers, too.
All the younger monkeys 5 learned to wash the dirty sweet potatoes to make them 6 to eat. But many old monkeys found it very hard to learn this and still ate the 7 sweet potatoes.
Then something very surprising . In the autumn of 1958, scientists found that the monkeys on other nearby 8 began washing their sweet potatoes too. Scientists still don't fully understand how this knowledge was 9 from one island to another.
