why should you only have one variable in an experiment?

Because if you have two variables or more, then when you measure the result of

the experiment, you won't know how much of it is due to each variable. The

answer to that question is the whole purpose of the experiment, so if you can't separate them, the whole experiment is worthless. That's why.

Because in a scientific experiment u are only trying to find one thing ... and thats your results ... the variable is what you need to know what is true ... and what is fauls