how to make a dropdown menu in html stack overflow

1                        <div class="main-menu">
2                            <ul class="menu-list">                                                  
3                                <li><a href="#">One</a></li>
4                                <li><a href="#">Two</a></li>
5                                <li><a href="#">Three</a></li>
6                                <li><a href="#">Four</a></li>
7                                <li><a href="#">Drop Down</a></li>                                
8                            </ul>
9