How can sand and gravel be separated?

Answer 1

Sand and gravel can be separated by simple screening - sifting through a screen that will allow the finer sand particles to pass through, while retaining the larger gravel particles on the screen.

Answer 2

By passing it through a filter, which traps the sand but allows the water to pass through.
That really depends on the circumstances; For instance if you are trying to remove the sand from water in a water bore various screens are available to achieve the desired result. Sand can also be removed from water by using a cyclone. Various filters do remove sand but need either constant cleaning or replacement. Sand can be settled out of water as well, as the sand is heavier than water.

Easiest is by filtering. A fine mesh or cloth would catch the sand particles and let the water through.

Or simply let the mixture settle. Sand is heavier, and will sink to the bottom, leaving a clear layer of water to be siphoned off from the top.

Or distill it. Bring the water to a boil, collect the steam. let it condense back into water. Sand doesn't boil at reasonable temperatures and would stay behind.

Centrifugation is used to separate sand from water. According to Wikipedia: "In chemistry and Biology, increasing the effective gravitational force on a test tube so as to more rapidly and completely cause the precipitate ("pellet") to gather on the bottom of the tube."

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You could do that. Or you could just run it through a paper filter.
The simplest solution to separate anything from water is to allow the water to evaporate and leave behind any solids in the solution; in this case, sand.
Boil the water until it has all evaporated and leaves sand left
Run it through a filter. e.g. towel

Answer 3

you sifter or put the rocks and sand on a screen and let the sand go through the screen
So you will have the rocks on top of the screen
And the sand on the table

Answer 4

Salts dissolve in water, but sand doesn't. Set the mixture in a strainer and run water through it.