Researchers have discovered that MP3 players like iPods and iPhones get fractionally heavier depending on how many music tracks they contain.

The difference between an empty iPod and a full one is only a 0.4 gram say the team from Blackpool University.

Tests showed that an empty 16GB iPhone weighed 133.2 grams compared to 133.6 for the same model filled with 3,500 songs.

Professor Roger Banks said: "This is the first time the weight of computer data has been measured. We all know it is made out of ones and zeros, but the ratio of ones to zeros seems to make a difference."

The boffins will now investigate whether the weight of different types of files music, video or pictures varies when they have the same file size.