If you're going to stick with something simple and limited in your gameplay, make it compelling; make it fast-paced; make it amusing; in short, make it the sort of game that immediately grabs you and forces you to hit 'replay' when you fail. Cows vs Aliens may be a simple game, but its magic comes from this simplicity and once you're done herding cows and destroying aliens you might just find yourself diving right back in to do it again.

The object of the game is simply to push (or 'herd') cows that enter in from the bottom of the screen in to a barn at the top of the screen. This isn't as easy as it sounds as tapping and holding on the screen generates a sort of force-field that pushes all the creatures nearby away from the finger in a large area. The force-field's basic application as a method of herding and keeping aliens away from cows is self-evident, but some players may be quick to note it has other applications such as sorting out aliens from cows by pushing them in opposite directions if you tap between them.

Collecting a set amount of cows refreshes the amount of time you have available while also increasing the flow of cows and aliens, making it harder and harder to create groups that boost your score in a large 'combo'. This could become a problem if you simply limited yourself to touching the screen with one finger, but multi-touch support is available and up to five force-fields can be used to help you to sort and score huge amounts of points.

If you're after something short and sweet with a bit of dark humor (seriously, who builds a barn on a cliff?) then you'd do far worse than checking out Cows vs Aliens.