My father loves jokes

and will often tell them to other people. When he discovers a new one he likes I will hear it a dozen or more times as he repeats it to family and friends. I was even foolish enough last week to buy a new (to him) book that happened to be in the local newsagent and the evenings have been punctuated with laughter. He tries to limit the number he reads so as to spread the pleasure.
Yesterday he read one as he was waiting for me to put lunch on the table and we had yet another discussion about what makes something funny. It is something that has fascinated and puzzled my father for years.
Humour is an elusive thing. There is of course visual humour, spoken humour and written humour. There are things which are only funny in context or in their own cultural setting. Scripts often fail to read as well as the action. There is humour which depends on our own knowledge -something particularly true of newspaper cartoons. Something which is funny in one language may not be funny in another language because it will depend on a pun or some other specialised linguistic usage. (What do you call a cat who works in a hospital? A first aid kit. You need to know about 'first aid kits' and the fact that a cat can be called a 'kit' - short for kitten. It's a simple enough riddle-joke in English but it would not translate well into other languages.
Pauline called in last with yet more grapefruit and more jars. My father told her the latest joke and mentioned the lunchtime conversation. She was reminded of an incident that had occurred to her some years previously, possibly because the colleague with her could not see what was funny. They were sorting out pictures for the hearing impaired children she once taught when Pauline came across a cartoon of a mother bear sitting with a small bear. Mother bear is reading to the baby bear and the beginning is, "Once upon a time there were three people...."
Humour turns things upside down. It makes us see the world differently - if we will let the humour do its work.