title: Function on created jquery droppable element
body: <p>Please tell me why the droppable function does not work on the added <code>drop</code> elements and how to solve this problem?</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>$(function() {
    $(".drag").draggable({
      start: function(event, ui) {
        $(".to_drop").append('&lt;div class="drop"&gt;New&lt;/div&gt;');
      },
      stop: function(event, ui) {
  
      },
    });
    $(".drop").droppable({
      revert: "valid",
      drop: function(event, ui) {
        alert(1);
      }
    });
  });</code></pre>
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.drag {
    width: 50px;
    height: 50px;
    background: blue;
  }
  
  .drop {
    width: 100px;
    height: 100px;
    border: 1px solid;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"&gt;&lt;/script&gt;
  &lt;div class="drag"&gt;&lt;/div&gt;
  &lt;div class="to_drop"&gt;
    &lt;div class="drop"&gt;Old&lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>