f(x)'s Sulli and Amber�revealed how they maintain their toned figures and the best part is that it's absolutely free!�

On the August 17 broadcast of 'Star King', the duo demonstrated how they lose weight by inflating a plastic bag and kicking it around in various ways so it doesn't drop to the floor. Amber also got into a hilarious mishap when her shoe fell off while trying to play the game.

SEE ALSO: f(x)'s Sulli to share her successful 'plastic bag diet' on 'Star King'