system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do our bodies decide where fat should go?
gpt
Think of your blood as a communal source of stuff for your body, and hormones as instructions being shouted loudly.

If you have a high blood sugar level, then your body releases insulin, this tells your cells to take up sugar from the blood ASAP as there is too much. Fat cells respond readily to this message, and suck up sugar from the blood to turn into fat to store for the long term. Too much sugar in your blood is really bad for you, so getting your fat cells to remove it and turn it into fat for long-term storage is a great thing to do.

Which fat cells take up the bulk of the sugar will depend on your genetics, gender, hormones etc. For some people it's the stomach, for others it's the bum. It's not such much that your body "decides" where it should go, that's like saying that a box of sweets "decides" to go mostly to the greedy kid at the party. No, all the kids grab some sweets, but the greedy kid grabs more on average, and becomes fat!

People who's fat cells are all "full" suffer from type II diabetes. This is where when your body has high blood sugar and shouts QUICK TAKE UP SOME SUGAR FAT CELLS, none of them do so, as they are all full. This is a bad thing.