The students are all delighted and now believe that the iron in the food is sticking to the magnet. But, like a good mystery, the story still has two more twists and turns.
To start, I hold up some nails and ask "does everyone likes to eat nails?" The kindergarten kids scream, "Nooooooo." The graduate students say "No," trying to look cool. I say I'm going to convince them otherwise.
I hand out a little magnet to everyone. I ask the class to figure out what it sticks to and what it doesn't stick to. After a few minutes running around the classroom, the kindergardners figure out that magnets stick to stuff with iron in it, and anything without iron in it doesn't stick. The graduate students sit there looking cool.
From behind the table, I pull out a box of Total Cereal (teaching is just like doing magic tricks, except that you get paid more as a magician). I show them the list of ingredients; "iron, 100 percent" is on the list. I ask by a show of hands whether this is the same iron as in the nails. 3 of 23 kindergarten kids say "yes"; 5 of 44 Harvard graduate students say "yes" (almost the same percent in both classes!).
I show the students that the box is sealed (and I have nothing up my sleeves), Then, I open the box, spill some cereal on a cutting board, and smash it up into tiny pieces with a rolling pin. I take the pile of cereal around the room and let the kids put their magnet next to it and see whether the cereal sticks to the magnet. To everyone's amazement, it sticks!
King points out that perhaps the cereal is simply sticky, like gum or tape. The graduate students clap their palms to their foreheads and think they have been duped. This gives King the chance to explain the importance of having a control group -- in this case a box of cereal without the iron fortification.
But, in the final twist, the control group cereal fails to stick to the magnet. It really is the iron in the fortified cereal that makes it stick to magnets.
Everyone gets to take home a cool fact (they love to eat the stuff in nails), I get to convey the point of the lesson in a way they won't forget (the central role of control groups in causal inference), and everyone gets a free magnet.Thanks to the excellent economics weblog Marginal Revolution for the link.