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 mum 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