Web Forms :: Swap Two DropDownList Values

How to swap ddl's selected value on buton click.E.g User has Selected One from ddl1 and three from ddl2 then when the user clicks the Button the values should be swapped.

User Selected

DD1 DD2

One Three

After Swap button click

 DD1 DD2

Three One

<asp:DropDownList ID="ddl1" runat="server">
<asp:ListItem>One</asp:ListItem>
<asp:ListItem>Two</asp:ListItem>
<asp:ListItem>Three</asp:ListItem>
</asp:DropDownList>

[code]...