"Use Diplomacy" joke

One summer, an old professor got a job on the railway as a steward. On his first day, he was accompanied by another steward to learn the ropes. "The job is really quite simple," said his tutor, "just remember to use diplomacy."
"What to you mean by diplomacy?" asked the professor, since that was something he'd never needed while teaching.
The steward-in-charge replied, "Watch me and I'll show you."
They proceeded down the train corridor, rattling compartment doors, opening them with their keys and offering tea or coffee. When the steward-in-charge flung open one door, before him stood a buck-naked woman. Without batting an eyelid, he calmly asked, "Tea or coffee, sir?"
The startled woman took a cup of coffee and he closed the door.
"Wow, did you see that cutie?" said the old professor excitedly. "She didn't have a stitch of clothing on! But, why did you call her 'sir'?"
"That's diplomacy!" explained his tutor. "I didn't want to embarrass her."
The old professor was very impressed with his teacher.
The following day, working on his own, the old professor flung open a compartment door and found a couple making love on the bed.
Remaining calm, he asked, "Tea or coffee, sir?"
"Tea," replied the man.
"And for your brother?"