Why should you have only one vaiable in an experiment?

Your experiment should show what the effect of a variable is on the outcome of the experiment. If you have two variables, you don't know which variable contributed what to the outcome.

For example: if want to see the effect that temperature has on water, you'll want to vary your temperature (temperature is a variable) and record your observations on the water. If nothing else is varied during your experiment and you do this experiment in your kitchen at sea level, then you may learn for instance that as you bring water temperature up from -60C to 0C it melts . If, however, you also vary your pressure (pressure is a SECOND variable) during this test, then you may find that if your pressure is 100Pa, as you bring water from -60C, it turns to vapor at ~25C!