Wyrd Sisters  - Terry Pratchett

It was on this cue that there came a thunderous knocking on the castle door. It seriously disturbed the castle porter, who was playing Cripple Mister Onion with the castle cook and the castle´s Fool in the warmth of the kitchen.

He growled and stood up. "There is a knocking without," he said.

"Without what?" said the fool.

"Without the door, idiot."

The Fool gave him a worried look. "A knocking without a door?" he said suspiciously. "This isn´t some kind of Zen, is it?"