The Controlled Variables: these are the variables that are kept the same in Tray #2. In other words, when you turn the lights up in Tray #1, the lights in Tray #2 stay the same. So when the tomatoes in Tray #1 get bigger, you can say that the difference in size between the tomatoes in Tray #1 and Tray #2 is the result of the independent variable, or the light that you made brighter.

Tray #2 is called a "control group." This is an identical setup to Tray #1, but it is kept in its natural, unchanged state. Tray #1 is called the "experimental group." So when you change something in the experimental group (like the brightness of the light), the control group stays the same so that you can measure changes in you dependent variable (the size of the tomatoes).