I received this one from Steve Poge who posts on alt.humor.puns

A man boarded the Metro Bus and was amazed to see a small dwarf seated on a pile of cushions, driving the bus. He was no more than two feet tall. He was wearing a green uniform, and a hat straight out of Robin Hood.

The dwarf stared directly ahead while driving, and kept repeating the same words over and over….. “tick-tock….tick-tock…tick-tock…” The man asked the person sitting next to him what the story was, and was told that the dwarf was a common sight on the Metro Bus runs, and was a great driver to boot.

“But why does he keep repeating the same words over and over?”, asked the man.

“Well”, his neighbor said, “I called the bus company myself to find that out. Seems the dwarf passed all the driving tests with flying colors, so they hired him right on the spot.”

“But why does he keep repeating the same words over and over?”

“Oh, that’s his job”, was the reply. “He’s a Metro-gnome”.