Verified Proof

Cards offering used textbooks for sale are posted on the college notice board at the beginning of each semester. One read: "Introduction to Psychology, $35, never used." The card was signed, "Must sell."

The next day a note had been added: "Good price. Are you sure it's never been used?" Signed, "Prospective buyer."

Below in a different hand was: "Positive!" Signed, "Professor who graded his exam."