You will need a long sand tray, propped up by a centimetre or two at one end, a sheet of cardboard, some sand and a large jug of water.

First, add sand to the sand tray. Then, at the higher end, build up the sand to make a'tableland' a few centimetres higher than the rest of the sand. Place a sheet of cardboard on this. This will be the waterfall lip of 'hard rock'. Hold the cardboard in place by covering it with about half a centimetre of sand.

Now pour water from the jug onto the upper end of the cardboard. It will run over the cardboard and sand and make a channel. It will then fall over the lip of 'card' rock and plunge onto the sand below.

Watch carefully what happens to the sand below the lip. Can you see a plunge pool forming? Is the lip undercut as the water splashes down? If not, you need to raise the level of the waterfall and pour more water.