Pour the salt water into a large pot. Fill it up about one-fifth of the way. Place the pot on the stove.
Place the glass jar in the center of the pot. Use a jar that is close to the same height as the pot, but not any taller. The jar can be as wide as you wish, just so long as it fits in the pot.
Cover the pot with aluminum foil. Make sure it is completely sealed around the rim of the pot. Use more than one piece if it is necessary. Gently press down in the center of the foil so it dips down. Do not push so hard that it breaks the foil.
Turn the stove on high heat, and let the water boil. The water will evaporate and rise up to the foil. As it gathers on the foil, it will turn back into liquid water. The dip in the foil will make the water move to the center and drip down into the glass jar.
Let it boil until the water stops bubbling. Turn the stove off and let the pot cool down for at least five minutes before handling it.
Take the aluminum foil off of the pot and take the glass jar out. The jar contains pure water, and the salt remains on the bottom of the pot.
