Cups and Downs
Posted on June 23, 2014 by Dave Youngs • 0 Comments
This week’s Puzzle Corner activity is a magic trick with a mathematical, as well as a slight-of-hand, component. I first came across this trick in one of Martin Gardner’s many books on recreational mathematics. I liked it so much that I have been stumping students, friends, and family members with it ever since. In order to make this trick work, you will need to practice it by yourself until the moves (illustrated at bottom) become automatic, before trying it out on someone else. Its success, like the success of many magic tricks, depends on diverting the audience’s in this case, your students’ attention. You can’t do this if you are uncertain of all the moves and take too much time making them. We’ll start by looking at how the trick is performed and then look at the mathematics involved.
You will need at least three cups to perform this trick. Plastic cups that are light and easy to turn over work best. When I do this trick in a classroom with students, I have as many sets of cups as I have groups. I quickly move from group to group doing the trick and leaving the cups there for students to ponder how it works. This works better than doing it once for the whole class.