the young salesman and the farmer

The salesman approached the farmer and began to talk excitedly about the book he was carrying. 

"This book will tell you everything you need to know about farming," the young man said enthusiastically.  "It tells you when to sow and when to reap. It tells you about weather, what to expect and when to expect it.  This book tells you all you need to know." 

"Young man," the farmer said, "that's not the problem.  I know everything that is in that book. My problem is doing it."