"Japanese arrowroot. It's an invasive species in some countries. If you make it into a soup, though, it forms a dilatant fluid." Heiji seemed more than happy to go off on this tangent now that he'd had even the slightest bit of encouragement.

"Basically, it acts like a liquid when you apply only a little bit of pressure to it. But if you strike it with enough force, it acts like a solid! Someone used that trick to make it seem like there was some spirit flittin' across the surface of a hot spring. Had a couple of journalists fooled, anyhow."