Wanna hear a joke? Of course you do:
I thought the dryer was shrinking my clothes. Turns out it was the refrigerator all along.
This joke could be passed along, indefinitely, for the rest of your life. You could tell it over and over at dinner parties and family reunions. This means that jokes, sometimes, are public goods.
What is a public good? A public good is something of value that is consumed with two distinct characteristics: non-excludability and non-rivalrous. Examples of public goods include parks, protection by the nuclear umbrella, and roads—to name but a few. They are public goods because it is hard to exclude others from benefiting from them.
A good is non-exclude if that it would be cost prohibitive to prevent non-payers from enjoying the good. For example, once I tell the dad joke above to my father-in-law, Jonathan, it would be extremely difficult and expensive to prevent him from sharing the joke with anyone else. What am I going to do, follow him around all day just to make sure he doesn’t share the joke at work, home, or the grocery store? Of course not. It would be too impractical, costly, and crazy. (Just as it would be cost prohibitive to put on a concert in the park, and attempt to prevent passers-by from hearing and enjoying the music).
What about the non-rivalrous characteristic? This simply means that more people enjoying the good doesn’t, by itself, increase the cost of the good, or take away from the enjoyment of those goods by customers. This clearly applies to the joke above: one could tell is endlessly, to different people, without thereby diminishing the enjoyment that anyone got upon first hearing it. If someone paid me to write the joke—nobody did, unfortunately—them sharing the joke with others wouldn’t take anything away from their enjoyment—it could potentially enhance it.
So, jokes can be a public good. Just not the kind of good we often think of as public.