Go to a YouTube video. Not necessarily all, but most normal types I've seen have it. I'm not exactly sure what the guideline is for it.

Here, have an example video to look at as I discuss this.

Now look along the bottom bar: you have play/pause, volume, timeline, resolution, SOCCER BALL . . . so yeah, click the soccer ball.

I've a feeling the 2010 World Cup is already legendary for this.