Why do you need a constant in an experiment?

In an experiment, you are only allowed to test for one variable at a time. This means that everything else must be the same or constant for only the one variable to be tested for. If say you were testing if plants grow better with more or less water, you need to keep everything else the same.

You need a controls and not controlled variable, you need a question you are trying to answer, then you want to come up with what you think will happen in the experiment