You shouldn't take food from others without asking.
John works at an office and is really hungry, and sees a candy bar sitting on the empty desk of a nearby coworker.
John wants to eat.
John waits until the coworker comes back and offers to give him a dollar for it.
John grabs the candy bar from the empty desk and eats it.
The coworker gives it to John for free, just asking that John return the favor every once in a while.
The coworker notices the empty candy wrapper in John's hand when he gets back and reports him to management.