A monk said to Jõshû, "I have just entered this monastery. Please teach me."
"Have you eaten your rice porridge?" asked Jõshû.
"Yes, I have," replied the monk.
"Then you had better wash your bowl," said Jõshû.
With this the monk gained insight.
When he opens his mouth, Jõshû shows his gallbladder. He displays his heart and liver. I wonder if this monk really did hear the truth. I hope he did not mistake the bell for a jar.
Endeavoring to interpret clearly,
You retard your attainment.
Don't you know that flame is fire?
Your rice has long been cooked
It's easy to attach a metaphorical meaning to Joshu's "Wash your bowl," and even somewhat useful ones. A popular interpretation is that, just as you wash out the residue of your meal so that your bowl is clean and available for use again, you must wash out your mind of accumulated knowledge and thought, so that it may be useful in whatever new endeavor you encounter. This definitely has ties to Zen, but it would be a mistake to stick to that interpretation and call it a done day.
The placement of this koan immediately following the Buddha holding up a Flower is no mistake, though, and it gains some new weight being partnered with that one. In the previous koan, the Buddha declares his teaching, giving it a gaudy, grand name, and announcing that it is being transmitted to one student, implying a sort of exclusive hierarchy, or at least establishing a clear delineation of "has" and "has not," which is of course, fallacious, and intended to spur investigation. Since an understanding in Zen can ultimately only come from oneself, and since "enlightenment" in the Zen sense is simply awakening to that self nature that you've always had, there can be neither exclusivity, nor transmission.
Here, we have Joshu taking the exact opposite approach. He refuses to make this ordinary self nature into something that he can transmit. He refuses to make it into something grand. Asked for teaching, and knowing full well that the realization must be the student's own, Joshu teaches him the only way he can; by showing his Zen in the context of the mundane. His teaching wipes away any sense of superiority. It completely discards any notion of the sacred. Zen claims that there is no merit; eating and washing your bowl are then just as holy as any ritual. How could they be any lesser, how could anything else be greater, when there is no merit to mark the difference?
It's also noteworthy that Buddha's Flower koan represents a more typical type of teaching in Zen, and Joshu's behavior in this case shows a very different approach that is very characteristic of what we know of him. In the previous Koan, the Buddha is setting a familiar kind of trap, leading his followers into getting lost in thought. Familiar, because this is what we see in many, many of the koans in this collection, and in many others. Many masters use that method to teach their students. Many shout, or hit, or intentionally confuse. Joshu is fairly unique in that he typically does not trap, and does not use aggression to get through. There's a saying that Nansen wields the sword that takes life, and Joshu wields the sword that gives life. Nansen is another Zen master, Joshu's teacher, and was fond of using the typical mental traps. His words will get you stuck, take away all of your logical options, and in doing so he awakens you to your true nature by means of removing everything else. Joshu's method is the opposite. He is not always easy to understand ("Mu"), but his answers are usually direct, simple, and completely lacking in guile. He openly gives what is needed, he doesn't conceal anything or misdirect. He opens his mouth and "shows his gallbladder," showing you exactly what he's made of, and pushes you directly toward an understanding.