system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where exactly does all the fat and especially excess skin go when we lose weight?

Does the extra dead skin just fall off at a higher than normal rate? How does the body "know" it doesn't need it anymore?

EDIT: I've marked this as "explained" because the first post did explain it for me, but there is not a secondary question that has popped up: "How much weight loss will result in excess skin on average?" Asked by raisinsmith.
gpt
Skin is flexible.  It stretches in all sorts of ways.

Fat is also flexible.  Fat cells simply get bigger. (although they are replaced, you don't have less than this or more than that, unless all the fat cells are full, in which case, more fat cells will be made... and then destroyed when you don't need the excess cells anymore.)

So when you lose fat, you're losing the contents inside the fat cell.  This fat is turned into energy.  Your fat literally 'melts' away from being burned as heat and energy and waste products.

Now, when the fat cells get smaller, the skin retracts.  Eventually, depending on the person, it can retract back to where it should be.

However, if the skin has been stretched for too long, or for way too much fat, then the skin's elasticity is not as great as it should be, so it will literally be hanging off you.  At this point, skin removal surgery is sometimes indicated.  Either for asthetic reasons (You don't like it) or actual health reasons (The extra skin is difficult to keep healthy because of it's hanging/folding about)

There ya go.