Any good small projects to impress 13 year olds?

   I've taken up a habit of making some tiny pocket-sized (or school bag sized) projects (one each week) and carrying it around with me to entertain Years 7 and 8 (12-14 year olds) and teach them some Physics at the same time. I've looked through some ordinary school demonstrations, but I need something small, great, not needing much preparation to show it, and also something none of them has ever seen, which is pretty hard...
   After a little more than a month I've already ran out of ideas, but I don't want to finish this practice, because both I and the kids like it.
   Any ideas???

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A Moiré pattern
 is a visual example of an interference pattern.

To make one you need two pieces of some sort of material with periodically spaced holes or slots, e.g. two combs, or two pieces of window screen. 
Then you put one piece on top of the other, and slowly rotate one of them.

Actually Moiré patterns would be trippy, and fun to make, and look at, even if they had nothing to do with physics. In fact it would be easy enough to just stare at them in wonder and not learn anything about physics.

But I think the lesson, if you want to try to explain it,  is this:  The Moiré pattern resembles a larger, more widely spaced (lower frequency), version of the pattern/signal that created it.

Basically what you're doing is taking two signals with frequencies that are close, call them f1 and f2, and producing a third signal whose frequency is the difference of the first two, Δf = f2-f1.

In radio, they call this trick heterodyning. The incoming, high frequency, signal is mixed with a locally generated (in the radio) high frequency  signal to produce a difference signal of low frequency, slow enough to hear.

A diffraction grating is a regularly spaced grid of closely spaced lines that can interfere with the closely spaced peaks and troughs of visible light, to produce an interference pattern with very wide spacing, like in centimeters, big enough to measure with ordinary tools like a ruler or tape measure.

Acoustic beating is another example.

Also aliasing:

Actually there are too many examples. So many that people won't take you seriously, and think you're just making it all up.  Still, Moiré patterns are still trippy and fun to look at.

a pocket jet turbine!
Try origami Small, cheap, easy to reproduce and depending on what you make can show physics. Just a thought.
What have you used so far ?

How about chaldni figures ? Speaker coil, plate, salt, 555 oscillator.

Same oscillator, piece of pipe = organ.

Cartesian diver ?

gruffalo child (author)  steveastrouk6 years ago
Cartesian diver is wet, not so easily shown on the floor in the hall, can't be stuffed into a schoolbag and they've seen it.
  We've had https://www.instructables.com/id/Make-An-Amazing-Magic-Ball/ , a bunch of red/green/blue LEDs ( they didn't know you can get yellow), streams of fake blood with hexocyanoferrate (unnerved the cleaning lady), ferrofluid, and the gravitation shield.
  I'm not sure I want electronics, because I'm not teaching anyone any further than resistors and lightbulbs, and I don't want them to look at it as something 'magical'. 
Any sufficiently advanced technology is indistinguishable from magic. Arthur C Clarke

Anyway, you're using electronics to demonstrate sound, not electronics per sae.