Gat a jar with a lid. Put some sand from a sand tray into it (about a quarter of the jar). Then add an equal amount of stoneless soil.

Add water to nearly the top, put the lid on and shake for a minute or so.

Now take the lid off and swirl the water around so that all of the sand and soil are in motion. Quickly pour the mixture out onto a large flat tray (one with a rim to catch the water).

Let the water evaporate overnight, or longer, as appropriate. Look to see if the soil has spread out further and made a thin, even layer (a floodplain deposit). What happens to the (heavier) sand?