There are more than half a million apps in the iTunes App Store, and you can find an app for just about anything.

Lots of apps are practical (alarm clock, anyone?); lots are helpful (the dictionary on-call); and lots enrich our lives and social connections in many different ways (Pandora, Instagram). And others are just plain… silly.

Some apps struggle to get any downloads, and others explode into global app stardom. Which apps sell like crazy? Theyâ??re often the silliest ones that leave us scratching our heads and asking, â??Why?!?â? Here are four of those apps.

The Moron Test

With a name like â??The Moron Test,â? you know this app is bound to be either incredibly silly, or incredibly stupid. Luckily for us, itâ??s silly, and despite the name, itâ??s genius. The Moron Test became a huge hit when it was released in 2009, and it spent weeks at the top of the iTunes charts. Since then, itâ??s remained consistently popular and is 11th best-selling app of all time. The Moron Test (99 cents) consists of several mini-tests that are seemingly simple, but their simplicity doesnâ??t mean theyâ??re easy. Youâ??d never think you would answer obvious questions like, â??Touch the orange apple,â? and â??Touch the smallest square,â? incorrectly you try this app. Itâ??s is quite the brain teaser, and the challenge of it is addicting – you wonâ??t be able to put it down until youâ??ve passed.

iFart Mobile

This app conjures up images of teenage boys laughing hysterically while making fart noises in the back of class as the teacher futilely attempts to teach Shakespearean sonnets. The silliness of immaturity, however, apparently makes for an incredibly high-selling app. For 99 cents, you can purchase an app that makes fart noises like, â??the burrito maximo,â? or â??the brown mosquito,â? with the press of a button. In 2011, for 21 days straight this app was the number one seller in the iTunes store, which at the time made it the 12th highest-selling app ever.

FatBooth

If youâ??ve ever wondered what you or your friends would look like after gaining 200 pounds, now you can find out. FatBooth is one of the silliest apps in the iTunes store, but itâ??s also one of the most popular of all time. Earlier this year, Apple released the list of the top 25 apps of all time. FatBooth was number 17 on the list of most popular paid apps. For 99 cents, you can turn any close-up photo of a face into the very fat twin of the person whoâ??s pictured. The app does nothing else, but millions of people were curious and silly enough to make this app a huge success anyway.

Angry Birds

On the topic of silly apps selling like crazy, Angry Birds is the definition of the subject. People all over the world have developed an obsession for this game, and itâ??s crazy how far the fanaticism has gone for such a silly app. Angry Birds is the most popular app in the iTunes store, period. It holds the position of number one in sales and downloads, and spin-off games Angry Birds Rio and Angry Birds Seasons made it into the top ten. For 99 cents each, these apps can entertain for hours, as you try to defeat the â??greedy pigsâ? who have stolen the eggs of the Angry Birds (if you donâ??t defeat them, God forbid, the Angry Birds will become extinct). With the precision of a slingshot, youâ??ll fling the Angry Birds into the air to topple the greedy pigsâ?? defenses.

This article was prepared with the help of the SEOmap team, a group of SEO consultants with a heart for mobile technology. You can learn more about their work here.

Free image courtesy of FreeDigitalPhotos.net