Liu Er, let me ask you what is cooking in this pot.

Liu Er, what do you mean by cooking in this pot?

This sentence comes from "Xi Weifu's Private Visit", the character in Liu Er's "Liu Er Zhen Mu", the owner of the drugstore. What's cooking in the pot? Liu Er said it was deerskin, and the emperor said it was horse skin. This is where the idiom "refer to a deer as a horse" comes from.