Professors are out

On a night a forgetful professor who return home late and weak. When re-opened the door, he realized that he did not bring the house key. Finally, he tap-strum her house door to his wife awakened and opens the door.
Because the house is very dark, at the time of opening the door of the wife can not identify the professor who is her husband. The wife said, "excuse me ...... professors are out . He does not have at home!"
Professor forgetful that said, "OK, if so I'll be back tomorrow. Thank you, Mother!"