In an old-time general store the owner installed a shallow 4 x 4 foot open top box covered with chicken wire. The bottom of the box had a dozen three-inch round holes with Mason jars screwed in from the back side. Each of the holes were marked 5%, 10%, 15% or 25%. Most of the holes were labeled 5%, with a few 10% or 15%, and only one was marked 25%.
The last Wednesday of every month was "Discount Wednesday" for any purchase that totaled more than $10. A mouse was released into the box, and whatever hole the mouse went down, that was the percentage discount the customer received.
Before the store opened on Discount Wednesday, the owner placed a few mouse turds in each of the 5% jars. Mice generally tend to go only where other mice have already been.
Thus, the game was rigged.