[Stuart]

An old one:

Four people are on a train. An Englishman, a Welshman, a young lady and an old lady.

All of a sudden, the train goes into to a tunnel, and the lights go out. Everyone hears a slap, and the train comes out of the tunnel.

The Old lady looks and the Englishman and thinks he slapped the young lady. The young lady looks at the Welshman and thinks he slapped the old lady.

The Englishman looks at the Welshman and thinks "I hope we go into a tunnel again, so I can slap him again".