What are some variables that can change in an experiment, and discuss the importance of a control group?

Question

This is a question my teacher assigned me in my biology class and I don't know the answer. HELP!!!

Answer 1

All the environmental conditions other than the experimental variable can change in an expeirment.  

In an experiment, you want to find out how one single thing affects another.  For example, how sunlight affects plants.  In order to see this, you must have two sets of plants, one set (the controls) kept out of sunlight, and the other set (the experimental variable) kept in sunlight.  You can tell the affects of sunlight by the differences between the two sets.  However, all the other conditions of the plants (i.e., the soil they are grown in, the amount of water they are given, their temperature and the availability of air are additional variables.  All of these must be the same for  BOTH the control and the experimental plants, or you won't be able to tell whether the differences between the two sets of plants are due to the presence or absence of the experimental (sunlight) or whether they are due to the other differences between the sets of plants.  

You must have a control so you know how the plants grow without the experimental variable as well as with it, just as a basis for comparison.