The questions are the same, but the answers have changed

When David Williams left the university, he went to Australia. When he returned to England for a visit twenty years later, he decided to go back to his old university.

David was delighted when he discovered that his old professor was still teaching there. He went to visit the old man, and after they had had an interesting talk, the professor went out to get something. While he was away, David saw an examination paper on the desk. He looked at the date on it and saw that it had been given to the students the week before. David picked the paper up and read it through.

When the professor came back, David said to him. 'Professor. I'm certain that these are exactly the same questions that you asked us in our examinations twenty years ago! How is that possible?'

'Yes, that's right,' agreed the professor calmly. 'The questions are the same, but the answers have changed.'