What is a good riddle, and what is the answer to the riddle?

What gets wetter and wetter the more it dries?" is an example of a tricky riddle that plays on contradiction. The answer is "a towel."

The riddle is difficult because there is an inherent contradiction within it. When people first hear that something gets wetter as it dries, they are usually stumped as this is apparently contradictory. Eventually, they realise that "dries" refers to an action that the object is doing, rather than something that is happening to the object. Therefore answer has to be a thing that dries another object. They eventually figure out that the answer must be a towel.