IIUC the easiest way to boil stuff without ceramics is usually in an animal's stomach or intestines. I believe you can also do it in a lined wicker basket.
I'm not sure exactly how these things are done but both of them seem much easier to figure out than how to fire a pot! (Requires very high temperature and a good understanding of the material to stop it cracking)
You have to invent a whole lot more to have a plastic bottle or even a paper drinking cup, at that point, it'd be simpler to just invent pottery, instead.
