Ask a professor a joke about switching to a plumber.

A doctor's water pipe is broken. He called a plumber to repair it. The plumber tinkered for an hour and finally repaired the pipe. He handed the doctor a bill for 600 dollars. "600 yuan!" The doctor said angrily, "I can't earn so much money in an hour as a doctor!" " ""yes. " The plumber said quietly, "I'm a doctor, too." "