See this link which I posted above for complete magic demonstration that I have created to amuse your class. I wrote it and am now moving it to the top of these replies so that your class & other will enjoy it. It is a simple trick. Costs nothing. And teaches economics. Your students may well remember how much fun it was to learn economics from you for the rest of their lives.
Disclaimer: Mark Twain (1835-1910-To be continued) is unlicensed. His river pilot's license went delinquent in 1862. Caution advised. Daily Paul ☑