Pour about three cups of the salt water into the mixing bowl.
Put the small bowl in the mixing bowl. Make sure none of the salt water gets inside the smaller bowl.
Cover the mixing bowl in plastic wrap, sealing all edges.
Center the small stone on top of the plastic wrap.
Expose the bowl to direct sunlight. Water droplets will form within an hour on the underside of the plastic wrap. The droplets should roll down the dip in the plastic caused by the stone and drip into the bowl.
Take off the plastic after a few hours and use the water in the small bowl, which should be salt-free, as desired.
Repeat the process to get as much water as you want.