system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If McDonald's and other fast food has so many calories in it, why doesn't it do as good a job of making us feel full compared to foods with similar calorie levels?
gpt
It's not the calories that make you feel full.  Your body has no idea how many calories are in something until it's fully broken down.  It's the volume that makes you feel full, and McDonald's is really calorie dense.