Quicksand is a colloid consisting of fine granular material, such as sand, silt, or clay, and water. It forms in saturated loose sand when the sand is suddenly agitated, and when water in the sand cannot escape, it creates a liquefied soil that loses its strength and can no longer support weight. Quicksand is a shear thinning non-Newtonian fluid, which means that when undisturbed, it often appears to be solid ("gel" form), but a less than 1% change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance, such as a person attempting to walk on it, the water and sand in the quicksand separate, and dense regions of sand sediment form, causing the viscosity of the quicksand to decrease suddenly. Someone stepping on it will start to sink, and to move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to reintroduce enough water to liquefy it. The forces required to do this are quite large, and to remove a foot from quicksand at a speed of 1 cm/s would require the same amount of force as that needed to lift a car.

Contrary to popular belief, quicksand is denser than the human body, and people and animals can get stuck in it, but they don’t get sucked down to the bottom—they float on the surface. If you do find yourself stuck in quicksand, the best idea is to lean back so that the weight of your body is distributed over a wider area, and slow back-and-forth movements can actually let water into the cavity around a trapped limb, loosening the quicksand’s hold.