how to make a dropdown menu in html stack overflow code example

Example: how to make a dropdown menu in html stack overflow

<div class="main-menu">
                            <ul class="menu-list">                                                  
                                <li><a href="#">One</a></li>
                                <li><a href="#">Two</a></li>
                                <li><a href="#">Three</a></li>
                                <li><a href="#">Four</a></li>
                                <li><a href="#">Drop Down</a></li>                                
                            </ul>