How to ensure a fair test ?

How to ensure a fair test?

By changing one variable at a time and keeping the others the same, we have a fair test.

Anything that affects the result in a test is called a variable.

For Example:

My Hypothesis - I GUESS that sea water will evaporate the fastest BECAUSE with the impurities present, there will be less water.

To test my hypothesis, I construct a fair test by placing 3 dishes of equal volume containing different types of water in the same location over a few days.

In this case, the only variable that I change is the type of water. The volume, exposed area, humididty level remains unchanged.