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I believe the term is "thixatropic", if I remember my old geology classes well enough. The idea is that if you make a thick slurry of "planar" particles, i.e. cubes or flakes, a small steady force allows the slurry to act as a liquid as the particles slide across one another aided by their flat surfaces and the available water.
Apply a sudden shock though, and the slurry becomes a rigid medium, because there is insufficient time to allow the liquid to act as a lubricant for the particles.
As mentioned above, a bowl of water with a sufficient amount of corn starch (flakes) creates such a slurry. Get the right mix and you can gently stir the mixture or even pour it out slowly, but if you punch the surface hard your fist will not penetrate. It's a pretty cool and cheap kitchen science experiment, and loads of fun.
I think this is basically how quicksand works too.
-dale
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