Good eye! (This is the bit between 0.40 -1.00 minutes, right?). I googled it, and apparantly milk contains a protein called casein, that when mildly heated, turns into a natural plastic. So this is really a thing, and you could even make your own plastics?! Example here: https://lifehacker.com/5760629/use-heated-milk-to-repair-your-cracked-china