Music producers Pharrell Williams and Chad Hugo have written so many hit songs, they have to time their release so they don't clog up the airwaves, writes Melissa Campbell.

Heard that absurdly catchy, slyly raunchy pop song on the radio recently? You know, the one about the milkshake that's better than yours?

Or about being a "slave 4 U"? The song about getting so hot you want to take your clothes off? Or perhaps the one that promises to have you naked by the end?

You're probably listening to a song by Pharrell Williams and Chad Hugo, the US music producers collectively known as the Neptunes.