Oobleck!

Here's a hands-on experiment that even the kids can try*.

  Oobleck (or corn starch in water) is a "Non-Newtonian fluid". When something tries to move through the oobleck, it tends to stiffen, and make it even more difficult for that object to move through. In the video below, sound waves from the speaker apply little pulses of pressure to the ooblek. Let's see what happens.

  When each pressure pulse (sound wave) is applied, the oobleck stiffens, and then relaxes when the pressure stops. This gives rise to the "fingers" of oobleck forming, then slowly wiggling around and breaking off. Look again at the video and watch the oobleck at the edges of the speaker. When the pieces break off and land on the part of the speaker that is not vibrating they stop stiffening and look almost like a normal liquid.

  Don't forget about the oobleck that's dripping off the edge too! It almost looks like a very thick syrup, similar to molassas. The same physics is happening here as well. As the oobleck slides down itself there's a force that causes it to stiffen up.

  Compare this behavior with plain water in a speaker cone (not responsible for musical tastes):

  Unlike oobleck, water doesn't get stiffer in the same way that oobleck does. If you have a bathtub full of water and you pull your hand through it it's difficult. That's the drag of water resisting you. Now if you drag your hand twice as fast it's just about twice as difficult. That's a Newtonian fluid, for those of you more advanced, it means that the drag of the water is linear in the rate of shear (how fast it flows over your hand).

   If we were to take our bathtub and fill it with oobleck (hypothetically, I wouldn't recommend doing this), we could pull our hand through it as well. Now if we were to drag our hand through it twice as fast, it resists us much more than twice as much. That is to say it gets stiffer as we increase the rate of shear, people call this dilatant.

  Alternatively we could imagine a material where if we were to pull our hand through twice as fast, it would actually be easier! This type of liquid gets softer as we go, or you may hear it as "shear thinning" or pseudoplastic.

  Oobleck is really a fantastically fun recipe and one that's so incredibly simple I really encourage you to do on your own: Mix 1 part water to 1.5 or 2 parts corn starch. Add some food coloring if you'd like. That's it!

  Another do-at-home non-Newtonian fluid recipe is that of Flubber.

*Always exercise caution when working with exposed electrical wiring, such as that which can be found in speakers, subwoofers, and other audio electronics. Children should always be supervised around electricity.