Cassie was taking two of her Grandsons on their very first train ride from Dayton, Ohio to Washington, DC.

A vendor came down the corridor selling Pop Rocks, something neither had ever seen before.  Cassie bought each one a bag.

The first one eagerly tore open the bag and popped one into his mouth just as the train went into a tunnel.  When the train emerged from the tunnel, he looked across to his brother and said:
"I wouldn't eat that if I were you."

"Why not?" replied the curious brother "I took one bite and went blind for half a minute."