This water bottle weight is quite a resourceful concept. If you consider that beginning a workout well-hydrated means that you won't require water until after you've finished, then why not use the mass of your beverage to help you build muscle?
Kyu-Hyun Lee has dreamed up an alternative cap to the standard PET plastic water bottle, made in the correct size to be screwed on securely. After you fill the container with H2O, you attach this handle-shaped lid instead of the original one, and the setup makes it easy for you to tote your drink on the run.
So basically, there are two benefits to the PETball: enhanced portability, and a dual function as a free weight. Use this as a water bottle weight to bring intensity to crunches, lunges and the like, and get the most out of your disposable container.