system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How can a soft drink like red bull or coke zero have no calories?

I mean, shouldn't they have some!? It's not just water. How does that work? Also, do such drinks mess up a diet by making your insulin shoot up or anything? 

I ask because I really like red bull but I'm on a diet, so I need to cut calories. This seems like a win-win, but I'm guessing its not. 

Thanks!
gpt
If there are fewer than 5 calories per serving they can round down. Next time you are at the store grab a can of the silver sugar free redbull.

The small? 0 cals.

The Medium? 10 cals.

The large? 0 cals.

The small is under 5, so they round down. The large is big enough to split into 2 servings, reducing the calories below 5 and permitting them to round down. The medium is too small to subdivide and they are left unable to round.