A friend and I were walking in the woods and spotted a bird in a tree.
I said, "Look how happy that bird is!"
My friend said, "What do you know about its happiness? You're not a bird."
I said, "What do you know about what I know about its happiness? You're not me."
My friend said, "What do you know about what I know about what you know about its happiness? You're not me."
Back and forth we went, following this line of reasoning to its logical conclusion.
Then the bird said, "Chirp!"