What is the Interface Segregation Principle, and why do 'fat interfaces' violate it?

Clients shouldn't be forced to depend on methods they don't use — a class implementing an interface should need every method on it, not just some. A 'fat interface' like Worker with work() and eat() methods forces a Robot implementation to provide a meaningless eat() method; splitting it into separate Workable and Eatable interfaces lets each class implement only what actually applies to it.