system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: 'Diet' drinks and how they have zero calories.

What is all that stuff in there and how is it, uh, nothing?
gpt
Zero Calorie drink are actually drinks that have less the 0.5 calories. They choose to round down to zero.

Calories are a measurement of energy in food. Not every part of any food item can be used for energy. For example Celery has a extremely low amount of usable energy because of all the cellulose in it (Human can not properly digest Cellulose) 

Diet Soda is mostly water, coloring, artificial flavoring and artificial sweetener. A common one is Aspartame which is 200 times sweet then sugar. This means you have to use so little of it that the total amount generates less then 1 calorie per drink.