To and fro

Put a layer of sand in a tray. It should be 5 or more cm deep. The tray must be at least big enough so you can get both feet in.

Now put enough water in to cover the sand.

Take your shoes and socks off (have a towel close at hand to dry your feet afterwards) and stand in the water.

Wiggle your toes about and see how easy it is to get your feet to bury themselves. This is because, when sand is under water, the water gets between the sand grains and makes it much easier for them to move about.

This is why sand moves easily in waves.