Fog

An old gentleman was lost in a London fog, so thick that he could hardly see his hand before his face. He became seriously alarmed when he walked in a slimy pavement. Then he heard footsteps approaching through the fog, and sighed with relief.

"Hallo? Where am I going to?" he cried anxiously.

A voice replied weirdly from the darkness beyond:

"Into the river, I've just come out!"