Why ViewState changes?

I have a ViewState["blah"] that is a list of strings. At some point I set a List<string> variable to be equal to view state. Whatever change I make to this variable affects the view state:
List<string> a = {"1", "2", "3"}; ViewState["Blah"] = a; .... List<string> b = (List<string>)ViewState["Blah"]; b.Remove("2"); Now, ViewState["Blah"] is {"1", "3"}. What am I doing wrong?
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