If you don't talk about the ice and charcoal, you will know the heat and cold(bīng tàn bù yán，lěng rè zì míng)

If you don't talk about the ice and charcoal, you will know the heat and cold

As an idiom, it means that the sincerity of the heart does not need to be expressed in action. It comes from the biography of Wang Shen in the book of Jin.

The word "B ī NGT à Nb ù y á n, l ě NgR è Z ì m í ng " refers to the sincerity of the heart without expression, which must be expressed in action. According to the biography of Wang Shen in the book of Jin, it is true that Yao, Shun and Zhou Gong, who are able to express their loyalty and remonstrance, have a sincere heart and a clear cold and hot quality

Chinese PinYin : bīng tàn bù yán，lěng rè zì míng