system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Assuming weight loss is purely about "calories in vs. calories out," how is it possible for the body to go into "starvation mode" and temporarily prevent weight loss from occurring

Hi all.  I'm currently on my own weight loss journey, and I've noticed that even though I've been strictly counting calories, I will go through periods where my weight refuses to budge.  The calories I'm consuming vs. calories burned should be low enough to cause weight loss but my body is somehow able to stave it off for days (or longer) at a time.  If weight loss is really just about "calories in vs. calories out" (as I've seen even Neil DeGrasse Tyson argue), how is the body able to keep itself from shedding the weight?  And, for the sake of argument, assume that I'm not making a mistake in my accounting.
gpt
Short answer... It doesn't.

Long answer, it stores a bunch of water.

So, your body has fat cells. Those cells store fat. When they're full of fat, that's it. They're 100% full fat cells. However, as you start losing weight and using those fat stores, those cells start getting smaller. So at say 50% full of fat, it's like a half deflated balloon. So as you lose weight you get hundreds of millions of half deflated fat cells.

Because our body is super efficient, it doesn't want to get rid of the cells yet because "we might need them again". But also half full cells are a waste as well. So the body starts replacing the Fat with water. Once the fat cells are completely full of water for a while, the body, being as efficient as it is, decides that maintaining these cells isn't worth the effort, we were holding on just in case, but it seems we're just wasting energy to do that. That's when your body starts getting rid of the water and the cells themselves.

Edit: TL:DR - Body replaces the fat in fat cells with water for a while before getting rid of the fat cells. (This is part of the reason it's important to drink a lot of water when attempting to lose weight)