system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If calories are just a unit of energy then how can things like Redbull have 0 calories but still give you energy?

I feel like there's some crucial bit of understanding that I'm missing here.
gpt
Energy is an odd word when it comes to food. You derive energy only from foods containing calories.  However, there are substances that for various reasons give you the perception of being energetic, or awake, or whatever. Consider that a pound of fat has approximately 3,500 calories worth of energy within it to use. That's enough to run for several hours. What things like Red Bull do is introduce chemicals into you that block receptors that tell you that you are tired. There are also other chemicals with no value, or little value, or that aren't really understood very well. Generally, it's the Caffeine and Guarana that are assisting in making you feel less tired. 

The rest is likely psychology, and later on psychological addiction. You'd be better served drinking a cup of coffee and avoiding the rest. Better safe than sorry.