My student asked me, "Do you know what four minus minus zero is?"
Sensing a joke, I said, "No. What?"
"It's poo," she explained.
"OK," I said. "Can you tell me why?"
The student came to the whiteboard and drew a diagram. I have attempted to reproduce it at right.
Then I laughed, because now the joke made sense.
You see, the final symbol is basically the Korean hangeul syllable glyph 똥 [ttong (or ddong)], which means "poo" or "shit."
[daily log: walking, 6.5]