Little Johnny and his friend Billy were on their very first train ride, with Billy's mother.

A vendor came down the corridor selling a candy bar that neither had ever seen before. Billy's mom bought each one of them a bar.

Little Johnny eagerly tore open the wrapper and bit a bit off into his mouth just as the train went into a tunnel. When the train emerged from the tunnel, he looked across at Billy and said: "I wouldn't eat that if I were you."

"Why not?" asked Billy.

Little Johnny replied, "I took one bite and went blind for half a minute."