Subwoofer / large speaker
Stiff 1/4" plastic / Plexiglass sheet
Large, transparent plastic container with lid (I used an empty Utz cheeseballs tub)
Heavy-duty trash bag
Sinewave source with amplitude and frequency control (computer with soundcard, or a signal generator, etc.)
Optional: Black fabric to cover up all your gear
How it works:
The cornstarch and water mixture, sometimes called oobleck, is a semifluid mixture with unusual properties. A normal fluid has a constant viscosity regardless of whether or how it is moving. But when the long, stringy starch molecules mix with water, they tend to become entangled, and resist pulling apart if pulled too quickly. So, the resulting mixture tends to stiffen up and become more like a solid when pushed around (that is, when a shear force is applied to it), and relaxes into a more liquid state when less force is applied. A fluid with this behavior is sometimes called a shear thickening fluid or more generally, non-Newtonian fluid.
As the membrane applies force nonuniformly, natural "harder" and "softer" spots will form in the mix. These will tend to be self-reinforcing as the hard spots provide more resistance against the membrane, pushing them to become harder still, while the surrounding softer material can flow underneath them, sending them essentially crowd surfing. These discontinuities are constantly dissipating and re-forming throughout the mix. The complex interaction of these areas produces a writhing mess that seems to have a mind of its own!
This project uses a speaker and flexible membrane (trash bag) to produce the shear. If you search the internet / youtube for things to do with Oobleck, one of the most popular is to put it directly into a speaker and watch it dance. This does work somewhat, but it is not ideal because a) Speaker cones are (by design!) rigid, so the cone does not so much impart shear as simply hop the mixture up and down, and b) putting wet stuff in speakers is really rough on speakers. Using the speaker to pump a flexible membrane sealed over it helps protect the speaker from damage, and by flexing at one of its resonant frequencies, the membrane imparts localized shear forces to give more angry, writing monsteriness and less vibration. By changing the frequency to hit a different resonance, the 'attitude' of the monster can be changed too.
The highlighter (or rather, the juice from it) is extremely UV-reactive and will make the Oobleck light up a bright radioactive yellow-green. Finally, the plastic jar helps minimize the acoustic noise from the speaker, deters kids from sticking their fingers in it (if you care), and helps keep the Oobleck from drying out.