Every year, on the first day of school, I subject my classes to a ruse in which I eat a “candle”. I get out a candle holder and put a candle inside it. I lead a class discussion in which we talk about the “observations of a candle” activity (which most of them have already done at least once), and I ask them what the flame is, what it needs, and why various means of extinguishing it work. Then I lit the candle and asked them how I should extinguish it. The most popular requests are “cover it” or “pinch it”. But this year in one of my classes, a student jokingly said “Eat it!” So I did, flame and all.
When I ate the “candle,” the student who made the “Eat it” comment was upset. “Wait! No! I was only joking! I didn’t mean it!” I had to explain not only that the “candle” was made entire from edible ingredients, but that I had been planning to eat it all along, and her comment created the perfect moment.
The edible “candle” was actually a piece of string cheese, and the “wick” was an almond sliver. (There are several variations on this–I’ve seen it done with a bore taken from an apple, banana or potato.) Like any magician, my effort went into getting my students to assume that it was actually a candle and move on to a discussion about the experiment that used it, thereby never giving them an opportunity to question their basic assumption that the candle was real.
After the demo, we had a nice discussion about the use of discrepant events in teaching physics.
