Telling a child not to run leaves open jogging, skipping and cartwheels. Telling them to walk does not. Positive phrasing is clearer, and it also keeps the day from sounding like a list of complaints. Spend an afternoon in a grocery store and count how many instructions children receive that only describe what they are supposed to stop doing.
This is the short version. The full lesson, with the story behind it, is in the book.
Take The Summer Camp MBA with you, or get the field notes by email while you work through the lessons here.