A horse walks into a bar and orders a pint of ale, The barkeeper says: "You're in here quite often, do think you might be an alcoholic?", "I don't think I am", said the horse, and then vanishes out of existence.
You see, the joke is about Descartes' philosophy of "I think, therefore I am, I am, therefore I think", but to explain this before the joke would be putting Descartes before the horse.
How to Kill a Rainy Day in London
In 1996 I had a flight from Heathrow (worst airport in the world) to Bombay, India (second-worst airport in the world) cancelled in the morning but could catch a flight in the evening. I did not want to go back home for a few hours only to repeat the trip later on the day, so I decided to kill a day in the tourist sites of London (even though I lived close to London). I had an inkling that I would soon move to the USA, so in a way this was a sentimental pilgrimage.
Bishop Berkeley espoused the school of thought called Idealism, which went along the lines of: "To be, is to be perceived". One fyne day he dreamt the following conundrum up: "If a tree falls in the forest and no one hears it, does it make a sound?" The implication is that if no one perceives the event, not only does the tree not make a sound, but the tree does not even exist.
This is, of course, a load of old rubbish. The good Bishop can't do me for defamation since he ceased being perceived in 1753. Had he lived in today's more cynical world, he may well have come up with more relevent thought-experiments to illustrate the philosophy of Idealism, thus: