Read the passage and accordingly, fill in the blank:
"What is sixteen and three multiplied?" asked the teacher. The boy blinked. The teacher persisted, and the boy promptly answered: "twenty-four", with, as it seemed to the teacher, a wicked smile on his lips. The boy evidently was trying to fool him and was going contrary on purpose. He had corrected this error repeatedly, and now the boy persisted in saying "twenty-four". How could this fellow be made to obtain fifty in the class test and go up by double-promotion to the first form, as his parents fondly hoped? At the mention of "twenty-four", the teacher felt all his blood rushing to his head. He controlled himself and asked again: "How much?" as the last chance. When the boy said the same thing obstinately, he felt as if his finger was releasing the trigger: he reached across the table and delivered a wholesome slap on the youngster's cheek.
The teacher controlled his anger because __________________.