CYou'll see if you don't use 'copy' in the setter the tests will fail.

gistfile1.m

@interface MyObject ()

@property (nonatomic, strong) NSString *myStrongString;
@property (nonatomic, copy) NSString *myCopyString;

@end

@implementation MyObject

- (id)init
{
    self = [super init];
    if(self)
    {
        NSMutableString *testString = [NSMutableString stringWithFormat:@"hello"];
        
        self.myCopyString = testString;
        self.myStrongString = testString;
        
        [testString appendFormat:@" world"];
        
        if(![self.myStrongString isEqualToString:@"hello world"])
        {
            NSLog(@"Expected string to be 'hello world', but was '%@'", self.myStrongString);
            abort();
        }
        
        if(![self.myCopyString isEqualToString:@"hello"])
        {
            NSLog(@"Expected string to be 'hello', but was '%@'", self.myCopyString);
            abort();
        }
    }
    return self;
}

- (void)setMyStrongString:(NSString *)myStrongString
{
    _myStrongString = myStrongString;
}

- (void)setMyCopyString:(NSString *)myCopyString
{
    _myCopyString = [myCopyString copy];
}

@end