Initially, you start out with simple cardinals. They can sing, but they aren't very powerful. You'll later unlock birds that can explode, drop eggs, accelerate, and split into three small birds. You'll utilize all of these skills to destroy the fortifications surrounding the little green pigs.

When launched, most of your bird types can only break through walls of ice, but you'll also encounter tougher pieces of wood and nearly unbreakable stonewalls as well. You only have a limited number of birds per stage, and you can't reorder the birds or choose the birds you'll use. This means you'll have to employ your specialty birds carefully to take out the sturdier fortifications, paying special attention to support structures and aiming.

Speaking of aiming, you do so by dragging your finger across the iPhone's screen and letting go when you've correctly angled the slingshot. Generally, it's much easier to angle the small slingshot than it is to reduce the amount you pull it back--it takes a few minutes to master the controls, but once you get the hang of it, you'll be flinging birds with ease.

That's not to say Angry Birds is easy. Managing your birds, hitting your marks exactly, and strategizing your target priorities makes the game challenging enough. There are more than three dozen stages to complete, with the difficulty increasing slowly. Even in the later missions, you'll encounter a less difficult level (likely one that involves dynamite), but then you'll get stuck for hours on a single puzzle.

Angry Birds is at its best when more than one strategy can lead you to victory. For example, on one stage you can launch your birds high in the air and have them break through the ice protecting the pigs. Or, you can launch all of your birds at the stonewalls surrounding the enclosure, hoping to knock the entire structure down. I found myself particularly frustrated with levels where there was only one very specific way to clear the pigs.