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              <h2>Click the Lights</h2>

<div onclick="switchLight(0)" class="light stop on" ></div>
<div onclick="switchLight(1)" class="light slow" ></div>
<div onclick="switchLight(2)" class="light go" ></div>

  Click on the lights and then look at the HTML. Notice the attribute I've added to each div. Then look at the JavaScript (JS) to see what's going on.
              .light {
  height: 120px;
  width : 120px;
  border-radius: 50%;
  opacity : 0.3; /* makes colors less bright by default */
  margin: 0 auto;
  display: inline-block;

.stop {background-color: red;}
.slow {background-color: yellow;}
.go {background-color: green;}
.on {opacity: 1;}

body {
  background-color : black;
  text-align: center;
  color :white;
  font-family: Helvetica;
              /* This line gets all the HTML elements with class="light" and assigns them to a variable called LIGHTS. */
var LIGHTS = document.getElementsByClassName("light");

/* This defines a function which turns off every light by removing the "on" class from the HTML elements.*/
function allOff() {
  for (var i=0; i<LIGHTS.length; i = i+1) {

/* This defines a function which first turns off every light and then immediately turns one on.*/
function switchLight(lightNumber) {
  var light = LIGHTS[lightNumber];

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