Everyone can describe quicksand: a person steps onto a solid looking surface which then turns to a mud soup, sucking the flailing person under, and leaving no warning for the next victim.  You’ve seen quicksand in movies such as Blazing Saddles, The Mummy, and The Neverending Story.   You maneuvered around quicksand in games such as the Mario and Zelda series and Prince of Persia 2.  Books, TV shows comics, anime, and even music videos have featured quicksand.  Examples can be found here: http://tvtropes.org/pmwiki/pmwiki.php/Main/QuicksandSucks

This idea of quicksand is wrong.  Science shows, like Mythbusters, have proved that people will not be pulled down to a muddy death, barring some exceptions.  Quicksand is formed when water saturates sediment and forms a suspension.  When a little bit of water is mixed with sand it increases the friction and allows the sand to be firmly packed, but as more water is added each sand particle becomes encased in water and the friction dramatically drops.  This causes the support force to decrease as well.

Your force of gravitational attraction, or weight, pulls us towards the center of the earth.  Your weight is generally well supported by the ground, but stepping on quicksand will cause you to “fall” closer to the center of earth.  Support force in liquid is called buoyancy, or buoyant force, and while it is not as strong as the surrounding ground quicksand does have this upward force.  Once an object’s weight matches this buoyant force the object stops sinking.

You probably don’t know the buoyant force of quicksand, but you can feel sure you won’t sink under because of density.  You can tell whether an object will sink or float by comparing buoyant force to weight or by comparing the density of a liquid to the density of an object.  Humans are less dense than water and less dense than sand, so you are still less dense than the combination of water and sand.  Just like in a pool, you will float in quicksand.

This does not mean there is no danger if you encounter quicksand.  If you wear a heavy backpack your weight and density are increased which could cause you to sink below the surface.  If you panic and flail around you will sink deeper which will make it harder to pull yourself out.  Exhaustion, dehydration, and exposure are very real dangers at that point.  So if you encounter quicksand you just need to stay calm and remember physics.