Madonna said she never knows which of her songs will be hits, and is surprised which ones become most popular.
Madonna thinks her most "retarded" songs become hits.
The 51-year-old singer always struggles to spot her future chart-toppers when she is recording an album, and is always surprised at the songs which go on to be fan favourites.
She said: "I've never been a good judge of what things are going to be huge or not. The songs that I think are the most retarded songs I've written, like 'Cherish' and 'Sorry' - a pretty big hit off my album 'Confessions on a Dance Floor' - end up being the biggest hits.
"'Into the Groove' is another song I feel retarded singing, but everybody seems to like it."
She added to Rolling Stone magazine: "Justin is a brilliant songwriter. I mean, 'What Goes Around... Comes Around?' Brilliant. I thought it would be a challenge to work with him.
"He's sort of a Cary Grant. I love him. I love working with him."