It makes the kids nervous. Doesn't help that I use my robot voice. I AM A MOLECULE I CANNOT STOP MOVING I AM A MOLECULE I CANNOT STOP MOVING I AM...
If I am feeling particularly brave, I get the students out of their seats, cram them in a corner of the room, then ask them to all do what I was doing. Chaos ensues, but within a minute or two, you have fairly random dispersion of the human molecules (except for the 3 or 4 males chasing Polly McMuffin), and I ask them to freeze.
I ask the lambs to look around--how did the particles get so spread out?
If I am feeling maniacally brave, I repeat the exercise, but now walk around with a lit candle, asking the students what they would do if they were heated up a bit. (Yes, I felt maniacally brave once, and hope not to feel so brave again, at least not until tenured.)
If I am feeling not so brave, I'll dilute a drop of milk in water, and have the kids look at it under a microscope. Once the fat droplets settle down, they wiggle and jiggle about, reflecting Brownian motion. Evidence of molecules in motion.
I know someone is thinking when a voice rises above the chaos--"but why do molecules keep moving?"
I can give a fancy pants answer, mumbling about the kinetic theory of matter, but when you push the issue, no one really knows. It wasn't that long ago that physicists argued whether molecules truly keep moving. Boltzmann, a prominent physicist pushing the idea that they do, committed suicide. I'm a passionate teacher, but I draw the line.
So in my best science teacher voice I say:
Students don't like that answer. I give the same answer when they ask why gravity acts the way it does (perhaps better phrased as why does mass act the way it does, but hey, these are freshmen).
Most just figure I'm clueless, but when surrounded by kids who have been told they're a bit slow enough times to start believing it, some find comfort that some things just aren't known, even by teachers.
If they only knew how true that is....
The "I am a molecule" routine was taught to me by Ms. Maria Rinaldi, my cooperating teacher back when I was still in the embryonic stage of pedagogy. I got a million dollar education from her.