Clyde and the Xorn
Once upon a time, there lived an eminently sensible earth pony named Clyde. Clyde was a sensible pony, but he had an extremely silly daughter named Pinkie. Knowing Pinkie’s frivolity, Clyde had instructed her to watch the fields, rather than assign her with a more important task.
One day, Pinkie came running up to him, frantic. "Father! There's a xorn in the south field eating all our rocks!"
Clyde galloped off to confront the xorn, only to find nopony there and his fields unravaged. His daughter began to laugh, rolling around on the ground in hysterics. "You fell for it!" she giggled.
Clyde approached his daughter with a stern look on his face, ready to punish her, when he saw a confused look on her face. "Why aren't you laughing, father?" she said.
"Pinkie," Clyde said, "allow me to tell you a story of the colt who cried timberwolf..."
Pinkie never tried to trick Clyde like that again, though she did put a whoopee cushion under his chair the next day.
The moral of the story is: Sometimes it is better to teach children than to punish them.