If you got stuck on an uninhabited island like Robinson Crusoe you will need to know how to get salt for cooking and drinking water for survival.
So, here you go:

What you need:
• large bowl
• short glass or cup
• tape
• plastic wrap
• small rock
• pitcher of water
• salt
• long spoon for stirring

What you do:
• Make saltwater by adding salt to fresh water.
• Stir the water until the salt dissolves.
• Pour about two inches of saltwater in a large bowl.
• Take an empty glass and put it in the bowl. The top of the glass should be shorter than the top of the bowl, but higher than the saltwater.
• Put plastic wrap over the top of the bowl. You may need to use tape to make sure the seal is tight.
• Put small rock right in the center of the plastic wrap, over the empty glass. That will weigh the plastic down and help you collect the water. Now you’ve made a solar still. It’s called a still because it distills, or purifies, water.
• Leave your still outside in the sun.
• Leave it alone for a few hours, or even a whole day. The longer you leave it out, the more water you’ll collect.
• When you’re ready to check your still, take the plastic wrap off and look at the water that’s collected in the cup. Do you think it’s salty or fresh?
• Taste it.

Tip: Rays from the sun heat up the salty water in the bowl. When the water gets warm, it evaporates and becomes a gas. When the gas rises and hits the plastic wrap, it turns back into water droplets. Eventually, gravity makes the water droplets roll down the plastic wrap towards the rock. Then the water droplets slide off the plastic wrap into the glass. The salt doesn’t evaporate, so it gets left behind in the bowl. Water evaporates in the same way from lakes, rivers, and oceans. The water heats up, turns into a gas, and then condenses to fall back down as rain.