> However, when you work with Fixnum literals, several things needs to be kept in mind:
>
> - Fixnums are immutable, there's no bang (!) methods to change them in-place like you can do for strings with update!()
>
> - Construct i += 1 in ruby is a syntax sugar for i = i + 1, so the result is a completely different Fixnum instance. Same is true for strings:
>
> s = "a"
> s += "a"
>
> After the above s will be pointing to another instance of String class with value "aa", while
>
> s << 'a'
>
> will be pointing to the same instance of String with updated value 'aa'
>

This explain makes sence. I've understood.
Thank you very much.