Pop Rocks and a Tunnel

Cassie was taking two of her grandsons on their very first train ride from Dayton, Ohio, to Washington, D.C.

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."