Smudge Jokes

Walter arrived at his office late one morning and was greeted with giggles from the pretty receptionist.
"What are you laughing at?" asked Walter.
"There's a big black smudge on your face," said the girl.
"Oh, that!" said Walter. "That's easy to explain. I saw my wife off on a month's vacation this morning; I took her to the station and kissed her good-bye."
"But what about the smudge?"
"As soon as she got on board, I ran up and kissed the engine."