This week, in the keynote of Nokia World 2010, we saw Rovio get on stage to detail how they were going to add content to their hugely successful game. The point was not that extra content was coming to Angry Birds, but that they had a route to adding extra paid-for content to an application.

The developers applauded, the journalists started frantically writing up that Mighty Eagle was coming to Angry Birds. If you have no idea what we are talking about, then you've come to the right place, because today's App of the Day is Angry Birds.

iPhone/iPod touch (reviewed) also Nokia, Palm and Android

If you head over to the Angry Birds website, you'll see that the game is listed as the number 1 paid for app in most countries you can think of. We've even brought you news that Angry Birds is heading to the console. That's quite a feat for a game that is incredibly simple. But simplicity is what seems to drive addiction when it comes to games.

The background to the story is this: a bunch of pigs, green pigs, have stolen your bird's eggs and made away with them. The plot, if there is such a thing, is to get those eggs back. What ensues is a series of linear platform-style levels that are essentially destructive puzzles.

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Each level presents you with a set of Angry Birds and some evil pig hunkered down in some sort of construction. You launch your birds into the middle of all this with aim of popping the pigs. To pass the level you have to pop all the pigs, but you only have limited birds, sometimes as few as one.

That means you need to use your noggin, look at the construction and figure out how best to get to the pigs. It's all about trajectory, speed and knock-on effects as you fire the bird in.

Different birds have different characteristics, from a straight-up basic bird, to explosive egg-dropping birds, to clusterbomb birds. Using a combination of birds you'll be able to strategically pop the pigs and make some progress.

Exactly why this is so addictive is something of a curiosity. It can be incredibly frustrating, a frustration compounded by those pigs that grin at you if you fail to pop them. But there are some moments of pure joy when you find the absolute sweet spot, send in your bird and bring the entire level crashing down and cashing in on the points.

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The game is divided into a number of different chapters, each with numerous games in that series. You'll find that you whizz through some and get stuck for a while on others, but the great thing is the re-playability. You are awarded stars for each level, so once you are some way into the game you might want to return to earlier levels to shoot for a higher rating.

There are also golden eggs hidden around some levels that take you through to some really special levels.

The best things about Angry Birds is that you get a lot of game for 59p. Really, you do, and its the sort of game you can dive in and out of when you only have a couple of minutes to kill, which pretty much makes it perfect.

What's more, once you reach the end, you get told that there will be more coming soon. Now that's a tease.

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