A game designed by a 14-year-old boy has topped the iTunes worldwide free app charts, ahead of the likes of Facebook and Skype.

Robert Nay, from Utah in the USA, created Bubble Ball, a "physics puzzle game" for Apple devices.

He learned how to code the game from a library book, after a friend's dad suggested he try to make an app.

On Wednesday (19 January), Bubble Ball had been downloaded two million times, according to Robert's figures.

It was also ahead of the free version of hit game Angry Birds.

"I think it's pretty cool because I never thought my game would do that well," Robert told ABC News.

"My friend's dad suggested I try making an iPhone app and I thought, 'Why not, that'd be pretty cool,' so I checked out a book from the library.

"When I saw that it was number one for the free apps, I was astonished."

He also says he plans to make more games, but his next project is "a secret".