歇后语【xiēhòuyǔ】

A 歇后语 has two halves. The first paints a scene, like the set-up of a riddle; the second gives the point. In writing a long dash usually joins them. They are spoken, humorous and folksy, and turn up in conversation, TV and informal writing rather than essays.

Logic or pun

In most, the second half follows from the picture: a basket can't hold water, so the effort comes to nothing. In others it is a pun. 外甥打灯笼 "the nephew carries the lantern" lights the way for his uncle, 照舅, which sounds like 照旧 "same as before".

Saying only the first half

Because everyone knows the second half, a speaker often stops after the first and lets the listener finish it. That is where the name comes from: 歇后, "rest before the end".

Dropping the first half doesn't work: 一场空 on its own is a plain phrase, and the picture that made the joke is gone.

Where they fit

A 歇后语 makes a point with a wink. In a report, an essay or anything formal it sounds out of place; there, a chengyu or plain statement is the right register.