Is This A Joke?

A priest, a rabbi and a minister walk into a bar. The bartender says, "Is this a joke?". ----That's it, that's all she wrote. I heard this one this week. I suppose there just might be barely enough here to meet the legal definition of a joke according the international office of weights and measurements. And I think it was Justice Potter Stewart who once famously said, "I know a joke when I see it". Yet, somehow it seems like there should be more.

I also wonder about a quick little one like, "I once lived in a town so small, the post office had a screen door". Is that legally a joke or not?

I can't help but look to other more developed examples such as: A mother and her young toddler son were down at the beach, when suddenly a huge ocean wave sweeps the poor child out to sea. The mother pleads with Heaven, "Lord, Lord. Please bring my son back. I'll never ask for anything ever again". God must have been moved by the mother's anguished cries to Heaven, so suddenly a second wave from the ocean sweeps the little boy back safely the to shore. The mom looks at the boy, and then looks back up to Heaven and says, "Yeah, but he had a hat!". To me, that's a joke. More development.

So the question remains, is the opening example a joke?