What is your favorite physics classroom experiment?

Back in secondary school I used to like messing with electronic circuits in physics but always preferred chemistry experiments because they were more visual.

We did a Bragg diffraction experiment with a scaled up crystal (a foam block with metal ball bearings in it) and a radiation source (microwave emitter) and we determined the structure of the ball bearings with intensity patterns. It was cool.

Anything that went bang.

Unfortunately, none of my HS or college physics classes provided that in the labs. In HS the focus was on optics and conservation laws: momentum, energy. So there were no bangs there. In college, I took quantum mechanics, mostly fancy math; so there were no big bangs there either.

So I did my own, at home, while in HS. My focus then was on rocket propulsion. I'd mix up black gunpowder and add various other fuels to it and then measure the thrusts. The purpose was to find the maximum thrust I could create by adding those other fuels to the gunpowder.

I found that adding ordinary shoe polish to the powder made the greatest thrust, which I measured using a fish weighing scale. My rocket motors were spent CO2 cartridges. It was a miracle I didn't blow off a hand.