关于搜索按钮与搜索栏之间的问题

怎么设置“搜搜看”这个按钮，当搜索栏中输入为空时，弹出提示“请输入关键字”
    protected void Search_Click(object sender, EventArgs e)
    {
        String theSel = RadioButtonList1.SelectedValue;
        if (theSel == "1")
        {
            Response.Redirect("ss_hp.aspx?ss=" + sstext.Text.ToString());
        }
        else
        {
            Response.Redirect("ss_hk.aspx?ss=" + sstext.Text.ToString());
        }
    }
}

<td colspan="2" bgcolor="#e9e9e8" align="left" valign="middle" style="padding-left:10px;">
            <span class="span11"> 搜 索：</span>&nbsp;
            <asp:RadioButtonList ID="RadioButtonList1" runat="server" BackColor="#e9e9e8" RepeatLayout="Flow" RepeatDirection="Horizontal" RepeatColumns=2>
              <asp:ListItem Selected=True Text="换物   " Value="1" />
              <asp:ListItem Text="换客" Value="2" />
            </asp:RadioButtonList>&nbsp;&nbsp;
            <asp:TextBox ID="sstext"  runat="server" Width="233px" />&nbsp;&nbsp;
            <asp:Button ID="Button1"  onclick="Search_Click" runat="server" Text="搜搜看" CausesValidation=false  />