= [NSMutableArray array]; // addObject: is a mutable-only method [anotherArray addObject: @"Hello World"]; 1) NSArray *myArray = [NSArray array]; 2) NSMutableArray *mutableArray; 3) mutableArray = myArray; 4) [mutableArray addObject: @"Hello World"]; Produces a compile-time warning, but works fine Line #3 produces a compile-time warning, Line #4 will BOMB at run-time