Bake 2 batches of cookies or other simple treat. Make one batch 1/2 the size individually to the other.

Before you start the lesson give each class member a small cookie (ideally on a napkin) and tell them not to eat it yet. The other batch is kept well hidden. Tell the class that they are not to eat that cookie until the end of class (I gave them a code word to avoid confusion). Tell them that they must not eat the cookie no matter what is said. The trick is that they must hold the cookie in their lap (or in reachable view)..

Throughout the lesson, throw in the odd "Why don't you eat that cookie?" "That cookie smells nice!" "Man, I'm hungry! Are you?" "Go on, eat the cookie, you get to eat it eventually anyway, why not now?" etc.