Mystring=Mystring2???

Hello!
I wrote these 2 codes:

sub x1
Dim Mystring As String
Dim Mystring2 As String
Mystring2=mid(Mystring, 6,3,"is")
MsgBox(Mystring)
end sub

and

sub x1
Dim Mystring As String
Dim Mystring2 As String
Mystring2=mid(Mystring, 6,3,"is")
MsgBox(Mystring2)
end sub

But, the result is the same, what wonders me!
In my first attempt I tried using just one variable, Mystring, which was ment in my mind to change itself by it sub, but this did not work.

So, the solution is in using 2 variables, but how then the result is the same, for Mystring and for Mystring2?

Many thanks!!!

:cry2:):tup