A Whack on the Side of the Head

I found this book in Borders in the Gardens right next to Mid Valley Mall. This book is on creativity, which is getting popular nowadays.

There is an exercise just to give you a chance to "think something different."

An eccentric old king wants to give his throne to one of his two sons. He decides that a horse race will be run and the son who owns the slower horse will become king. The sons, each fearing that the other will cheat by having his horse run less fast than it is capable, ask the court fool for his advice. With only the words the fool tells them how to make sure the race will be fair. What are the two words?

I crack my head for a long time. What do you think is the answer?