You can break it down to the proverbial lemonade stand. Set one up and let the kids run it. Or when the kids run a bake sale to raise money for the dance - there's a good opportunity to impart this information.

Using real money will pique their interest.

Probably the best episode of "The Apprentice" was the very first, where the teams had to each set up and run a lemonade stand for a day.