Build a fire that's small enough that the water won't boil over when it's placed over the fire. The fire should be built so the large pot can sit level on the embers.

Place the 1-gallon pot directly on the embers. Place the 1-quart pot in the center of the larger pot, with the medium-sized rock in the quart pot. The rock must be heavy enough to keep the smaller pot from floating once the saline water is added to the larger pot. Fill the large pot with sea water until the water is just below the brim of the inner, smaller pot.

Take the plastic sheet and loosely, but completely, cover the top of the larger pot. Tie the string around the plastic and the pot to make a complete seal. Place the smaller rock in the center of the plastic sheet so it is directly over the center of the inner pot.

Monitor the still so the water does not erupt into a rolling boil, which may upset the inner pot. As the salt water heats, notice how small drops of condensation gather on the inside of the plastic sheet. This is clean water that should run down the depressed plastic sheet and drip into the inner pot.

When the water in the outer pot is completely evaporated, remove the still from the heat. The salt from the saline will remain at the bottom of the inner pot. The process can be repeated until you have enough water.