how to show message and tooltip in lable

hi

I have lable in page:

<asp:Label ID="Lblorder" runat="server" CssClass="lblorder1"></asp:Label>

that it will show session value:

        if (Session["totalorder"] != null)
        {
            Lblorder.Text = Convert.ToString(Session["totalorder"]);

        }

I want if there wasn't any data in session in tooltip of lable it will show "there isn't any order"

How I can do it?

Best regards

Neda