You can find out by getting a bag of builders sand from a DIY store and also a packet of sandwich bags.

Fill the bags half full and seal the ends with sticky tape. (Do not fill them completely because you need room for the sand to be able to move about in the bag.)

Then stack a pile of them across the middle of a washing up bowl. Throw them down firmly one on the other because you want the sand to fit together snugly.

Add water to one side of the bowl. Does the other side remain dry?

To find out why bags are used, pile up loose, damp sand across the center of the bowl. Add water to one side of the bowl. What happens?