system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we get hungry every few hours, even when we haven't burnt all the energy/calories from our previous meal(s)?

I'm sure there's something very obvious that I'm missing, but I don't get why humans get hungry all the time even if there's plenty of energy left to burn. For instance, if we eat a breakfast that's 1500 calories and then sit on the couch all day and only burn maybe 700 calories, why do we feel hungry again a few hours later, even though there's still 800 calories left? If I had to guess, I would guess that the feeling of hunger (like stomach growling)is a technique our body uses to tell us that we need more food and more energy. But why does our body make us feel hungry even if there is leftover food that we haven't burned yet?
gpt
We get hungry when our body is about to start the process of warming up the furnace to burn fat.  

There may be plenty of calories available in our bodies, enough to last for days, but we get hungry when we run out of the "easy to access" energy and our blood sugar level gets low.  

Blood sugar levels have a certain safe range -- too low and we're weak, too high and the blood turns toxic.  When we eat, blood sugar levels rise as food gets digested and sugars get added into the bloodstream.  If it rises above a certain point, our body starts skimming off the top and putting the excess into fat.  If it falls below a certain point, we get hungry, and if it falls even further, we start burning off that fat to return those sugars to the bloodstream.   

Hunger fades once the fat is burning and sugar levels rise.  When the fat calories run out, we just get tired not hungry because the body is already in "fat burning" mode.  

As soon as we eat anything (edit: anything with any sugar), our body immediately stops burning fat and switches to using the incoming food instead.  When that runs out, you get hungry again as part of flipping the switch back to fat use.