When water evaporates from the ocean, it leaves salt behind. If you had no fresh water to drink, you could distill (or purify) ocean water by taking advantage of evaporation. Here's how:
1. Add salt to two cups of water and stir until it dissolves, then pour it into a large bowl.
2. Place a short glass in the middle of the bowl. (This glass should be shorter than the rim of the bowl, but taller than the level of the saltwater.)
3. Now cover the bowl with plastic wrap, taping the edges, if necessary, to get a tight seal. Place a small rock or other weight on top of the plastic directly over the glass in the bowl. This helps you collect the distilled water in the glass.
4. Put the bowl outside in the sun. Leave it for several hours, or for the whole day. When you check it again, there will be water in the cup. Taste it to find out of it's salty or fresh! (You can also use electricity to test it for saltiness by making a saltwater circuit.)
The sun warmed the water in the bowl until it evaporated, becoming a gas. When the gas rose and hit the plastic it condensed there in droplets (just like water vapor condenses into clouds). The droplets rolled down the plastic toward the weight and eventually fell into the glass (like rain falling from the sky). The salt was left behind in the bowl, making the water in the glass pure enough to drink.
Build a Saltwater Circuit to test your purified water for salt without tasting it.