A Virginia farmer was trying to train a small horse for a saddle-horse
for his daughter, and was riding the animal up and down the road past a
haystack. In order to accustom the horse to sudden fright, he directed
his son to hide behind the haystack and jump out as he rode by and say,
"Boo!" The boy did so, and the horse reared and plunged till he had
thrown the rider on the roadside and ran away. The old man picked
himself up, cut a switch from a handy hedge, and was about to chastise
the boy. When the boy expostulated, declaring that he had only done what
he had been directed to do, the old man said, "Yes, I know you did, but
you let out altogether too big a Boo for such a small horse!"