"Don't throw sand." becomes "Sand can really
hurt if it gets in someone's eyes." or "If sand gets on our floor,
the floor gets very slippery. We don't want anyone to slip and fall."
"Rulers are for measuring." or "Rulers are for drawing
straight lines." (Or "If you
do that again, I'll use that ruler to measure you for a coffin.")
"If covers are bent back, it
will ruin the binding."