Half Of What

Master Kwan sat in front of the student, poured the pitcher into his glass, and stopped.
“Is the glass half-empty or half-full, Stinkbug?” asked the teacher.
The student scratched his recently-shaved scalp. “It is full, Master,” he said.
“With what?” asked the teacher.
“It is half-full with water,” said the student. “And half full of air. Half plus half is whole.”
“Drink,” commanded the teacher. “Fill the glass with air.”
The student drank. “Delicious,” he said, smiling.
“It is not water, Stinkbug,” said the teacher. “It is poison.”
“Then it is a delicious poison,” said the student, and he died.