You shouldn't drink other people's beverages.
Greg is exercising at a gym and wants some water when he sees someone's water bottle that was left behind an hour ago.
Greg wants to drink some water.
Greg walks several minutes to the water fountain in the gym and drinks from it.
Greg grabs the abandoned water bottle and drinks it all to quench his thirst.
The owner of the water bottle returns and drinks the water, quenching his thirst.
The owner returns shortly after and tries to drink the water, only to find it empty, and is disappointed.