I can tell you how to make quicksand, but don't be too quick about it. After you make it I'll show you some experiments ya can do to understand quicksand better.
First, what IS quicksand? Sand which is 'merely wet', even saturated from rainfall or runoff, is not quick. A mud puddle, no matter how deep, is not quick. Instead, it is the action of the water bubbling up from the BOTTOM of the sand deposit-not the top, as in rain; or from the nearby surface, as with a puddle-that makes quicksand. More specifically, quicksand occurs when a sandy/silty deposit overlies a flowing spring. The pressure of the water beneath the surface keeps the sand particles suspended. Some use the word 'colloid' to describe this state. If you have seen spaghetti or rice in a pan at a slow rolling simmer, you have an idea of what is going on beneath the surface of quicksand!
Most natural quicksand deposits are very small in size...several feet to maybe a dozen feet across. It is their depth which makes them frightening, for quicksand is the worst of both worlds for one who steps into it. It offers no support-much like stepping into water, you sink right down, even over your head. On the other hand, quicksand is more viscous and dense than water, so you feel trapped and it is very difficult to quickly pull yourself out. I will return to this point later in my essay.
So why would you want to make such an obnoxious substance as quicksand? Hopefully not to get rid of your enemies, although in medieval and colonial times, stashing a criminal in the local bog was a favorite form of capital punishment. Instead, lets make some quicksand just to explore its rather bizarre, and dangerous, properties.
My favorite approach is to get a large metal bucket, say 15 or 20 gallons at your local hardware supply store. It is better to have a deep bucket, rather than a wide one. Take a hose and place the nozzle end in the bottom of the bucket. It helps if you can position the nozzle pointing upwards, but this is not essential. When the water is turned on later, the pressure will push the water upward through the dry sand anyway...which is the effect you wish to create.
In any case, place the nozzle in the middle of the barrel and begin filling the bucket with sand, or perhaps a silt/sand mixture. Cover the hose to a depth of at least 12 inches, preferably more...2 feet is ideal. Now, turn on the water at low moderate pressure. The water will percolate UP through the sand, because of the pressure from the hose. This is the exact opposite of what happens during rainfall or runoff, when gravity prompts the water to percolate downwards, and it is this difference that makes the sand 'quick.' Eventually the bucket will fill up and overflow the sides...make sure the pressure is not so great that it flushes the sand grains over with it.
Notice how the sand has no ability to support any weight whatsoever. A small rock placed on the surface will sink quickly beneath it, and fall to the bottom eventually. This is because the the sand grains are help in suspension-in a colloidal state-by the action of the rising water.
You can simulate the effect quicksand has on a person who steps in it. Take a small stick about an inch or more in diameter and push it vertically into the sand. Allow it to sink in briefly. Now grab hold of the stick and try and remove it very quickly. Notice the feeling of being 'pulled back' or 'sucked under' that you feel? Pulling the stick creates a small vacuum in the sand beneath the surface, since the sand has some viscosity which is greater than water and takes a few moments to fill. This vacuum pulls back as it tries to fill itself in with sand, water, and the end of the stick, much like the one you use on your carpet pulls up the dust and grit.
Now repeat the process and try and remove the stock very slowly. You will not notice any 'sucking' or vacuum effect. That is because you are moving the stick slowly enough that the sand can fill into the empty voids. Thus the recommendation outdoorsmen have always given to those who fall into quicksand. DON'T PANIC....move slowly, in a swimming motion, You will be able to swim out of the quicksand pit and reach the edge, or grasp vegetation and pull yourself out.
Turn the water off at the spigot, and very soon after, the quicksand disappears. The wet sand eases to the bottom and becomes firm, if not exactly solid; and any excess water rises to the top under pressure. In nature this water would flow away or evaporate. In any case, you can now see that the sand is no longer quick. The muddy mess will support the rock, whereas it did not before.