Why cant you draw a conclusion about the effect of one variable in an investigation when more than one variable is changed

In an investigation or experiment you are looking to see the effect ONE variable has on the subject. It won't be successful if you have more than one variable since the results won't be valid. For example, if I wanted to see the effect different types of fertilizer has on the same plant the variable would be the fertilizer since I will be using different types. If I water one plant every day and the other plant every other day then the water is now another variable because it's not the same. Hope this helps!