What are the only things that can change in a valid experiment?

You can change any variable in an experiment (the amount of water, sun, type of light, fertilizer etc.) but for the experiment to be VALID only one thing can be changed for the experiment.

Example: I'm trying to see if the plant will grow better with more or less water so the only thing that changes is the amount of water. Everything else will have to be the same (type of plant, fertilizer, dirt, container it grows in, amount of light etc.)