Why is it necessary to test only one variable in a controlled experiment?

because if you test more than 1 you wont be able to tell which one has caused the change in your experimentWhy is it necessary to test only one variable in a controlled experiment?
All other conditions should not vary.Why is it necessary to test only one variable in a controlled experiment?
Because if you changed two variables in your experiment and got a different Data than from your control you would have no clue which variable affected it and to what numerical degree.





Example: a metal bucket with 8cubic meters of water weighs 10 kilograms


a wood bucket with 6 cubic meters of water weighs 8 kilograms





but we don't know what made it weigh less the bucket material or the less water