Pour 2 inches of milk, vinegar or oil into a bowl.
Place a short glass into the center of the bowl with the opening facing up. Make sure that the glass is shorter than the bowl.
Cover the bowl with plastic wrap.
Place a small rock into the center of the plastic wrap. This will cause the plastic wrap to lean in towards the opening of the glass.
Place the covered bowl into direct sunlight. When the bowl heats, the water from the liquid will begin to evaporate. This evaporated water vapor will be caught by the plastic wrap, and will fall into the glass as fresh water.