Why doesn't the color change on click?

Question

css

https://jsfiddle.net/puckk4bv/13/

Answer 1

$(document).ready(function(){
//
});

Answer 2

On onload and syntax. On the JSFiddle where you choose the library, set Load Type : OnLoad

$('.but1').click(function(){
  $('.first').css({"background-color": 'yellow'});
  $('.second').css({"background-color": 'green'});
});

Although if one property is already possible