hoisted on his own petard =joke 🌽🌽🌽

"If there are any idiots in the room, will they please stand up", said the sarcastic teacher.
After a long silence, one freshman rose to his feet.
"Now then mister, why do you consider yourself an idiot?", inquired the teacher with a sneer.
"Well, actually I don't," said the student, "but I hate to see you standing up there all by yourself."