The 'Story of My Life' singers, who reveal their secrets in the December issue of Seventeen magazine, say the 20-year-old hits the gym more than any of the other lads and has a great body.
Niall Horan, also 20, said: "Payne-o! That fellow is ripped. He works hard at it though, so I'll give him that."
The boys, who have just finished their mammoth world tour performing in more than 130 countries, say singing isn't just kept for the stage for the 'X Factor' hunks.
When asked if they warm up their vocals in the shower, singer Harry said: "We all do! We can sometimes hear Liam through the wall."
The 'Best Song Ever' star, who is dating school-friend Sophia Smith, also divulged details about his grooming routine and items he can't live without while on tour.
He said: "Toothpaste. Got to have fresh breath, especially when we're singing in each other's faces."
Heartthrob Zayn Malik, 20, said lotion was important, adding: "You have to stay smooth."
Louis Tomlinson, 21, also revealed his true colours, after being called the practical joker of the band.
He said: "I've been labelled the prankster. I change the words of songs when the guys least expect it. Their reactions are priceless."