system
Given the following post, respond as if you're commenting on Reddit:
human
How can caffeine be calorie free but still give me energy?

A calorie is a unit of energy, and caffeine gives me energy, so how can it be calorie free?
gpt
Caffeine doesn't "give you energy". It is a stimulant that reduces your body's ability to react to a chemical (adenosine) that makes you feed drowsy. You then believe you feel more awake and alert.

There is a major irony here, though: caffeine increases brain metabolism rates, but simultaneously decreases the blood flow. So you may not be making the best decisions, or learning and performing particularly well, though you think you are perfectly functional.