The Cliff-notes version of what makes funny funny suggests something like this: humor arises from transgression and surprise. We have expectations of what's normal, so humor involves challenging or overturning those expectations in a way that is mildly alarming and/or absurd.
A duck is not supposed to walk into a bar, in the normal course of affairs. The duck's smart-alecky comment is usually something of a surprise or something patently absurd. <insert sound of canned laugh-track>
Radiolab, the fantastic radio show, also looks at gelotology in this episode.
National Poetry Month -- it's doesn't have to be serious, people.