Non-newtonian fluid

Quicksand is an interesting material. In can act as solid or liquid. Quicksand is a soupy fluid made up of sand or earth and often water. If forms when something causes sand, earth, clay to have reduced friction and fluid-like properties. Typically it's something is water. The water in quicksand separates the individual particles and reduced friction. Sometimes quicksand can forms without any water like an earthquake. An earthquake can shift the ground such a way that reduces the friction between grains of dirt or sand.

How to escape? With quicksand the more you struggle the faster you sink. To move within the quicksand you must apply enough pressure on the compacted sand to bring enough water to liquify it. So if you relax, your body will float on it because your body is less dense than the quicksand. The human body has a density of 62.4 pounds per cubic foot which means you can easily float more easily on quicksand than on the water. The problem is that quicksand traps people. A motion will loosen the grains of sand and reduce friction.

Oobleck (cornstarch mixed with water)
Quicksand is a non-newtonian fluid. It's viscosity changes based on how much force is being applied and what speed the force is applied. This is why quicksand is difficult to escape. Unfortunately, cornstarch in water is very similar to quicksand. Because they are both oobleck and quicksand change viscosities based on the amount of stress that is being applied to the substance. At times they act like a solid and other time they act liquid. Oobleck and quicksand have another similarity because they can both trap people.

Non-newtonian fluid
Oobleck and quicksand are both non-newtonian fluids (a fluid that does not follow Newton's law of viscosity).  This means that how they behave changes based on how much stress is being applied. When there are a lot of stress non-newtonian fluids will become viscous and resemble a solid. When a non-newtonian fluid is under very little stress it will often have less viscosity and act more like a fluid. This is why you can roll the oobleck and turn it into a ball but when you release it, it appears to melt. This is also why it's difficult to escape quicksand because the faster you move the more quicksand will act like a solid.