You need a deep tray (a small fish tank, for example) and some gravel from a garden shop. You also need a piece of perspex sheet to fit inside the tank.

Put the perspex sheet against one side of the tank.

Half fill the tank with water and add a centimetre or so of gravel to the bottom. Now rock the tank back and forward sharply so that waves form in the tank and the gravel starts to rub against the perspex.

You should see scratch marks on the soft perspex after a few minutes.