Albert Einstein was once giving an Exam paper to his graduating class of Physics students. It turned out that it was the exact same exam paper he had given them the previous year. His teaching assistant was alarmed at at what he saw and thinking it was the result of the professor’s absent mindedness , alerted Einstein.

‘Excuse me, Sir,” said the shy assistant, not quite sure how to tell the great man about the blunder.

“Yes?” said Einstein. 

“Umm, eh its about the test you have just handed out”

Enstein waited patiently.

I’m not sure Sir, if you realise it, but this is the same test you gave out last year. Its identical”. 

Einstein paused to think for a moment, then said “ Yes , it’s the same test but the answers have changed”.