Quicksand is a colloid hydrogel consisting of fine granular matter (such as sand or silt), clay, and salt water. The origin of the name refers to "quick" in the older meaning of "alive" rather than "fast," and is thus similar to the origin of the term quicksilver for mercury. Quicksand is a non-Newtonian fluid: when undisturbed it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After the initial perturbation—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to suddenly increase. In order to move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it.

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