In the past, there was a man who kept thirty sheep.
One morning, he discovered that one sheep was missing. Where had it gone? Upon examination, he found that his fold was damaged. During the night a wolf had broken into the fold and devoured the sheep.
His friend said to him:
"You had better mend the fold quickly."
"The sheep had already been devoured by the wolf. What is the use of mending the fold?"
He did not listen to his friend's advice.
The following morning, he discovered that during the night the wolf had again broken into the fold and devoured another sheep. Not until then did he realize that his friend was right.
Consequently, he mended the fold at once, and thereafter no more sheep were devoured by the wolf.