Edit in JSFiddle

$(document).ready(function() {
  $("button").click(function() {
    $("#content p").animate({fontSize: '20px'}, "slow");
  });
});
<h2>jCombat Demo - jQuery Events</h2>

<div id="content">
  <p>This is a paragraph.</p>
</div>

<button>Click me increase font size</button>
#content p {
  background-color: yellow;
}