Usually a control group is a set of conditions that are "nominal" or "normal" meaning, in your case, "break kept in a normal household environment."
 
Think then about what "a normal household environment" means - temperature, humidity (moisture in the air), exposure to air.  You can probably think of other things like that but stay away from contrived variables (like "how many times my little brothers drops the break in a bathtub")
 
If you think about just one of those - temperature - you might get curious about whether or not bread kept in a warmer or cooler situation would grow mold faster or slower.
 
You might ask yourself, "how can I make bread last longer without molding?"  Answers to that question - the things you would change - are your variables.
 
That's one way of looking at an experiment.  One type of bread, different environments.
 
There could be an entirely difference set of variables, however, based on what goes into the bread, the ingredients.  How salt content or this type of flour versus that type of flour would affect the rate of mold growth.

Your variables are what breads does mold grow on faster, during heat or cold and your control group is the level of moisture. Having different breads/heat/cold/moisturer content is the experiment.