Question: So I have this script that is a counter that follows an infinite animation. The counter resets to 0 everytime an interation finishes. On the line fourth from the bottom I am trying to invoke a x.classList.toggle() to change css when the counter hits 20. When I replace the classList.toggle with an alert() function it works, but as is no class 'doton' is added to 'dot1'. What am I missing?

<http://jsfiddle.net/8TVn5/>

```
window.onload = function () {

var currentPercent = 0;

var showPercent = window.setInterval(function() {

    $('#dot1').on('animationiteration webkitAnimationIteration oanimationiteration     MSAnimationIteration', function (e) {
  currentPercent= 0;}); 

   if (currentPercent < 100) {
   currentPercent += 1;
  } else {
    currentPercent = 0;
 }

   if (currentPercent == 20){document.getElementByID('dot1').classList.toggle('doton');}
    document.getElementById('result').innerHTML = currentPercent;
}, 200);

};

```

Answer: 
You can't append a `list` to a `tuple` because tuples are ["immutable"](http://docs.python.org/2/library/functions.html#tuple) (they can't be changed). It is however easy to append a *tuple* to a *list*:

```
vertices = [(0, 0), (0, 0), (0, 0)]
for x in range(10):
    vertices.append((x, y))

```

You can add tuples together to create a *new*, longer tuple, but that strongly goes against the purpose of tuples, and will slow down as the number of elements gets larger. Using a list in this case is preferred.
You can concatenate two tuples:

```
>>> vertices = ([0,0],[0,0],[0,0])
>>> lst = [10, 20]
>>> vertices = vertices + tuple([lst])
>>> vertices
([0, 0], [0, 0], [0, 0], [10, 20])

```