As he sat down on his designated seat, he noticed that there's a small saucer with an off-white round thing on the table of every seat.
Having attended numerous sit down dinners, he assumed that the thing is butter.
When the dinner was about to start, a waiter came by with a small jar and started pouring some liquid onto the saucer.
The butter started expanding vertically until it looked like a tower.
That's when he realized it's a hand towel.
When his host sat down opposite him, he told him that he thought the towel was butter and almost wanted to eat it.