In a survival situation, making a solar still will allow you to obtain and purify water using the sun and gravity. You can produce drinkable water from snow, urine, salt water, cut vegetation, etc.
A solar still works in the same way that nature makes rain. The sun will heat the water until it evaporates and becomes a gas. This vapor rises and condenses on a surface and turns back into water droplets that can be collected. Impurities, salts, microbiological organisms, and other contaminants are left behind.
Weather you are creating a solar still with a cup and a pan, or digging a pit into the ground, knowing how to put together a solar still is a valuable survival skill.