Brains contain millions of bits of knowledge. Some of these bits are connected to each other, like how Catholicism is connected to John Paul II, or a hammer is connected to a saw. These connections aren't complete; it is not exactly a full mesh of facts; rather, webbing with holes in it.
Example: The nodegel. At current count, there are about 950,000 little nodes. Each one gets connected through various forms of links. Suppose one is at Node A, and then searches for Node B. Those two are now connected with a link.
A joke is funny because it creates a connection between two nodes never before realized in any individual's mind. The more different these nodes are, usually the funnier the joke, until the nodes are so far unrelated so the joke isn't funny, just weird.
The joke is told and everyone is laughing; the connection is made. There are people out there who add on to the joke by restating it, hoping to get the same laugh. They are simply retracing the same connection that has already been made. If they told the joke before the first man told it, there would be no connection and the joke may be hilarious.
Over time, the connections we don't normally use fade off until the joke that was told with the nodes that made said connection is just as funny as it was five or ten years ago.