Why does reverse also flips my other dynamic array?

[Namal via Digitalmars-d-learn]

import std.stdio;

void main(){
  int [] a = [1,2,3,4,5];
  int [] b = a;

        writeln(b, " ", a);
        a.reverse;
        writeln(b, " ", a);
}


I get:

[1, 2, 3, 4, 5] [1, 2, 3, 4, 5]
[5, 4, 3, 2, 1] [5, 4, 3, 2, 1]

Why is also b flipped here? This doesn't happen if I use static 
arrays.