An Absent-Minded Professor

An absent-minded professor was moving to a new house further along the same street. His wife knew that he was prone to forgetting things and so she wrote down the new address on a piece of paper he went off to college. She handed him the paper and the key to the new house and reminded him not to go back the old address.
 
That morning, one of his student asked him a complex question and the professor wrote the answer down on the back of the slip of paper. This student asked whether he could keep the paper.
 
Forgetting what was on the other side, the professor said, "Certainly."
 
In the evening, he returned out of habit to the old house, tried the key and could not get in. Realizing his mistake, he search in his pockets for the slip of paper with the new address, but off course there was no sign of it. So he wandered along the street and the stop the first personable-looking lad whom he saw.
 
"Excuse me, I'm professor Galbraith. You would not happen to know where I live, would you?"
 
"Sure, dad," said the boy.