I enjoy the sort of scientific demonstrations we might call "illustrations:" they make visible something about sound, or heat, or motion that would otherwise pass unseen. Here is an example from the field of acoustics.
Out of the Cabinet of Physics come brass plates, of various shapes and sizes, each supported on a single leg at its center. An object like this can be persuaded to make a sound—one might thwack it with a spoon, perhaps, and hear it chime. In today's video, it is more effective to stroke a plate with a violin bow.
Sound is produced by motion. When stroked to produce a tone, some parts of the plate move more than others.
We see pale sand being sprinkled onto the dark plates. A sand grain that lands on a rapidly-moving portion of the plate will be jiggled, and will bounce to another location, perhaps to be bounced again. If it lands on a location that is scarcely moving, it will remain there, safe from further jiggling.
By this process, grains of sand tend to accumulate in the special locations or "nodes" that are not moving as the plate continues to vibrate with a particular tone. The spiky patterns of sand reveal the special geometry that's connected to that tone. These patterns are named for Ernst Chladni (1756—1827).
If the strokes of the bow excite a different resonance, making a higher note, the Chladni figure changes as the sand grains rearrange themselves along the new nodes. Different shapes and sizes of plate also yield different Chladni figures. Even a paper membrane, not touched by the bow but excited only by sound passing through the air, exhibits its own Chladni pattern. Normally the processes that create sound occur invisibly, but in this demonstration, something about sound has become visible.
By the way, Ernst Chladni is renowned for his contribution to acoustics, but he might be familiar to the scientists of the Vatican Observatory for a different reason. Chladni argued in 1794 that certain iron-rich minerals we now know as meteorites might originate beyond the Earth. It's a startling idea, but as evidence was gathered over the next few decades, other scientists came to accept that Chladni was correct.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.