<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js"></script>

<div class="box"></div>
.box {
            width: 150px;
    height: 150px;
    margin: 30px auto;
    background-color: blue;
}
$(function() {
  $('.box').mouseover(function () {
    $('.box').css("border-radius",'50%');
  });
  
});
	

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.