Ever since I read a little of Arthur Koestler's ideas on creativity, I have been interested in humour. He said that science, the Arts and humour all depend on a collision of ideas - but they collide in different ways. In science, one idea can combine with another to produce a better explanation of what is happening. For example in one model of an atom it is compared to a ball, and inside the ball is a tiny central round nucleus and electrons spinning around it in layers. Of course an atom is only like this to a certain extent, and as scientists discover more, the less useful this model becomes. However, the moment that such a suitable analogy occurs to a scientist, is the Aha! or eureka moment.
In the arts, I suppose a metaphor accomplishes a similar purpose, it describes things in a different light. But whereas in science the concept is a concise explanation; in the Arts there is a sense that it extends a concept to include more, often emotion. This is what Koestler called the Aaah moment. For instance I suppose I could say that 'his soul was as empty as an atom; a dense angry mass at its centre'.
So that was the idea behind the Aha and the Aaah moments, and Koestler postulated that humour was a variation on this theme - part of a continuum. According to what I remember, Koestler said that humour was a clash of ideas. There had to be an underlying logic, but then this was taken to an inappropriate extreme. So, taking the above example, I think that if I extend 'his soul was as empty as an atom; a dense angry mass at its centre,' with 'and all the kindnesses bestowed upon him fell towards it but were unable to settle; instead they were condemned to orbit, like so many electrons.' then it becomes humorous. At least in my opinion. The metaphor has been extended to such a ludicrous extent, it has become funny. Koestler called this the Haha moment.
Added later: Frank Wilson has a much better example here.
Looking in the index in this book I see no reference to Koestler, but just flicking through I see some interesting cartoons, so I shall look forward to reading this little book and finding out another viewpoint on how, when and why jokes are funny.