Sometime back Google introduced video annotations in YouTube that can either be plain text appearing over the video or they can be links which point to other videos hosted on YouTube.
Now this may seem limiting because you cannot link to external content from YouTube videos but Hexolabs, a mobile game development company, found a creative use for annotations.
They have developed a very basic but probably the first video game for YouTube using just the annotations feature. Called "A Car’s life" you can play this YouTube game here or via the embedded clip below:
The rule is simple – a button will momentarily flash on your screen while the video is playing – you have to click that button before it disappears else your car meets a crash. If you click at the right moment, you jump to the next level.
This is where it gets interesting. The next level is actually another YouTube video clip that is linked to the button you just clicked in the previous level. Good idea.
Related: YouTube Tools Guide