Question

Explain why you can only change one variable in an experiment

Answer 1

The purpose of an experiment is to LEARN the EFFECT of something.

The way you do that is to CHANGE the thing and see what happens.

You can change as many things as you want to.  But If you change
TWO things and observe the result, then you don't know which one
of them caused the effect you see. 

Or maybe BOTH of them working together caused it.  You don't know. 

So your experiment is not really much good.  You need to do it again.