A solar still uses the sun to help collect water. To make a still you need a spade (or something you can use to dig a hole), some leaves and rocks, a clear sheet of plastic and a receptacle for the water. For this experiment to work it must be a sunny day and the hole should be dug in a sunny spot.
To make an effective solar still you need to dig a hole about a metre wide, and 50cm deep. The experiment will work with a smaller hole, but you won't collect as much water.
Place the collector in the centre of the hole and pack leaves or mown grass around it. Make sure you leave a few centimetres between the top of the collector and the cuttings and the top of the hole.
Place the clear plastic sheet over the top of the hole and pull it flat, securing it in position with large rocks or stones. It should not be touching anything except the edges of the hole. Take a small stone and place it on top of the plastic sheet, above the cup. The sheet should dip down over the cup. Leave the solar still for a few hours.
After some time you should see condensation on the inside of the plastic sheet. Water in the ground and the leaves has evaporated and condensed on the sheet. These small drops of water will congregate over the cup, due to the dip in the sheet caused by the stone. The droplets will fall into the cup and water will collect.
If you were trying to survive in an inhospitable environment you could drink this water, but remind pupils that it is not advisable to do so in this experiment, as it is easy to get dirt in the water by mistake.