I completely agree with this. Duck-laden object oriented programming tutorials made no sense as I tried to transition from C to Objective-C.

The best explanation I found was Chapter 2 of Apple's "Object-Oriented Programming with Objective-C":

Every object has both state (data) and behavior (operations on data). In that, they’re not much different from ordinary physical objects. It’s easy to see how a mechanical device, such as a pocket watch or a piano, embodies both state and behavior.

http://developer.apple.com/library/mac/documentation/cocoa/c...