"A freshman will put absolutely anything in his pocket."
Thus spoke my geology professor of yore, explaining why he never let his prized mineral specimens out of his sight. I recalled his dictum years later while lecturing on solid-state chemistry (really materials science) to a class of some 300 MIT freshmen. I had by then been lecturing enough terms to realize that the uninterrupted drone of my voice would put the class to sleep within 30 minutes. I therefore tried to interject a joke, a gag slide, or a bit of experimental magic to liven things up.
I had heard of numerous lecture demonstrations over the years but had not worked out the details. One semester I had a first-year graduate-student teaching assistant named Thomas Eagar, who was a cracking good experimentalist. I gave him my recollections of these various demos and asked him to do what he could with them. He made them all work. I still have his notebook. (Tom raced through to a Ph.D., quickly became full professor, and a little later became my boss as department head.)
A major part of the course was devoted to alloy phase equilibria and to the various reactions which may occur in the liquid and solid states. It often comes as a shock to freshmen to learn that atoms in solids diffuse around so as to obtain energetically favored surroundings. This diffusion becomes appreciable on a human time scale at about half the absolute melting point and is extremely rapid just below the melting point, where atoms change atomic sites at about a hundred million times per second.
Just above 273C an aluminum-75 wt. % Zn alloy exists as a uniform solid solution. At lower temperatures the alloy decomposes into Al-rich and Zn-rich phases. The reaction proceeds on cooling, hence by Le Chatlier's Principle, involves a release of heat. If the alloy is quenched from above 273C and refrigerated, diffusion is suppressed and the reaction will not occur. My assistant cast the alloy into attractive medallions about 3 inches in diameter and refrigerated them.
One day I was giving the students a discussion of alloy solidification. About mid-lecture I paused and passed around a half dozen of the refrigerated medallions to show examples of ingots. Sure enough most of the medallions disappeared into freshman pockets. Body heat was enough to set off the latent transformation, and the hotter the medallions got, the faster they got even hotter, quickly reaching a discomfort zone.
Larcenous students were soon squirming uncomfortably in their seats and yelping all over the lecture hall. I then explained just what had happened, much to their chagrin. In this one demonstration I not only roused the sleepers but simultaneously illustrated autocatalytic phase transformations, solid-state diffusion, and the importance of honesty.
I might not repeat this trick today, what with the litigious nature of current society. Actually, even though the incident took place decades ago, I might still be sued by former students.
This bit of classroom magic got me written up in a recent book on college pranks entitled, If at All Possible Involve a Cow. The butts of most of the pranks were professors or deans, and sometimes rival fraternities or other universities. My prank, or in MIT parlance, "hack," was the only one with students as victims. Touché!