ELI5/How does beef stock become thick of fat and water don’t, usually, mix?

Question

Like how does cooking it down make a thick Demi glacé or how does one get a gelatinous broth/stock?

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Answer 1

The fat doesn’t mix with the stock. When you make stock, fats float to the top and you either skim it off or put it in the fridge and remove the solidified fat. Stock gets thick because you convert the collagen in the connective tissue into gelatin. That’s why when you make stock you generally make it from joints