Why is it important that you test only one factor at a time when you perform an experiment?

Answer 1

This is one of the basic principles of the scientific method. Let's say you do an experiment and you change two things at once. How do you which thing you changed affected the result of the experiment? By keeping everything exactly the same as before and changing one thing at a time, you know that any change in the result of the experiment was due to the change in that one thing.

Let me use an example. Let's say you are measuring how fast you can drink water (don't ask me why you would do that experiment!). So you could vary a few different things to test... like change the water temperature from hot to cold, change the shape of the container you are drinking from, or maybe even how thirsty you are! But if you want to know the affect of the temperature of the water (can you drink a cold glass of water as fast as a room-temperature glass or a glass of hot water?), you don't want to change from a cup to a squeeze-style bottle at the same time. If changed both things at once, you wouldn't know whether you could drink it fast because of the temperature or because of the container, right? But if you compare different temperatures water, all from the same container, you know that whichever you can drink faster was because of its temperature!