Menu控件不能弹出子节点

Menu控件使用时出现不能弹出子节点
例：
父节点    父节点    父节点    父节点
子节点    子节点    子节点    子节点
子节点    子节点    子节点    子节点
子节点    子节点    子节点    子节点
子节点    子节点    子节点    子节点上面是想出来的效果。
可是实际效果是：
父节点    父节点    父节点    父节点
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Menu ID="Menu1" runat="server" Orientation="Horizontal">
            <Items>
                <asp:MenuItem Text="父节点" Value="父节点">
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                </asp:MenuItem>
                <asp:MenuItem Text="父节点" Value="父节点">
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                </asp:MenuItem>
                <asp:MenuItem Text="父节点" Value="父节点">
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                </asp:MenuItem>
                <asp:MenuItem Text="父节点" Value="父节点">
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                    <asp:MenuItem Text="子节点" Value="子节点"></asp:MenuItem>
                </asp:MenuItem>
            </Items>
        </asp:Menu>
    
    </div>
    </form>
</body>