In brief, you have someone stand on one foot with both arms extended sideways. You press down on one arm, and they tend to fall over. Then, you give them a gizmo of some sort (I used a rubber band), and repeat the test, and people find they don’t fall over anywhere near as easily. A video is below; another quite good one is here.
The trick is a very slight difference in the direction you press. To make them fall off, you press slightly outward from their body; to keep them balanced, you press every so slightly towards their body.
I did this in class, having seen the video but never having done this before. I went through four or five different volunteers, and was able to get the effect every time.
Nobody believed that my rubber band was the cause of the difference. When I asked people for ideas as to what was going on, several of them could be easily tested on the spot. One person suggested I had placed my hands in different locations in the two tests. I got a new volunteer, placed my hands very precisely in the same spot, and was able to tip or not tip them at will.
One suggested a placebo effect. Problem: None of the people I gave the rubber band to believed the rubber band was the cause, which made it hard to argue that it was a placebo.
We also talked a bit about the mechanisms that some bracelet makers propose for how their bracelets are supposed to improve balance. Some invoked things like “natural energy fields” or “life energy,” which was a great lead in to talk about how living things differ from non-living things. Do living things have some sort of energy that non-living things don’t?
Eventually, one person near the very front of the class figured out what I was doing. I think that with a little practice, though, it would be very difficult to detect.
In summary, this is easy to do, requires nothing but a student volunteer, and is a great jumping off point for all kinds of discussions about hypothesis testing, evidence, mechanisms, and lots more.
That you get to push people around is just a bonus.