What are these "riddle jokes"?
歇后语
KEY

≪xiehouyu≫, witty folk simile (popular witty or humorous simile consisting of two parts, of which the second is the &quot;punch line&quot; and is often left unsaid because the listener is assumed to be familiar with it)

We have this in English as well:
•Make like a banana—and split
•Make like a tree—and leave
(Although I'm not sure what we call these in English)
A Chinese-English Dictionary

a two-part allegorical saying, of which the first part, always stated, is descriptive, while the second part, sometimes unstated, carries the message, e.g. 泥菩萨过河-自身难保 like a clay idol fording a river-hardly able to save oneself (let alone anyone else)

The Chinese Wikipedia entry for xiehouyu has a 粤语常见歇后语 and 客家话歇后语 sections - so while some xiehouyu certainly are regional - and every topolect has them -  the ones in dictionaries or that are &quot;well known&quot; will be understandable everywhere.