A hot, grey-green, sour drink of fermented mung bean starch water, sipped with crisp fried dough rings and shredded pickles.
Douzhi is the liquid left over from making mung bean noodles, soured by fermentation and simmered. Old Beijingers drink it at breakfast and say it settles the stomach; newcomers usually find the smell hard going at first.
It's often held up as the test of whether you're a real Beijinger. The Qing court is said to have taken to it too, though the stories vary.