To compare how changing one variable among two or more objects has an effect, all “variables,” or factors, must remain identical. When the variables in a science experiment remain the same for one object or set of objects, they are called the “control group,” since the variables are controlled. The objects in which something is varied from the control group are said to be in the “experimental group”. For example, if you want to compare how sunlight affects the growth of a plant, you need two identical plants, and only the amount of sunlight each receives should be varied. All other factors, such as room temperature and the amount of water they receive, must be kept constant.
When you buy a package of flower or vegetable seeds, the package tells you how deep in the soil to bury the seeds. In nature, however, seeds could end up at any depth, depending on human, animal and weather activity. Hypothesize that if seeds are buried too deep they will not produce plants.
Fill six wide-mouth jars with potting soil. Obtain a package of quick germinating seeds, such as radish or bean seeds. Plant two seeds at a time in each jar, in case one seed is not viable and does not germinate. Lay two seeds on top of the potting soil in the first jar. In the second jar bury 2 seeds 1 inch deep. In the third jar bury the seeds 2 inches deep. Similarly, bury seeds one inch deeper in each successive jar. Use a pencil to poke a hole in the soil to drop seeds in each jar. In this project, the controlled variables are temperature, light, location and the amount of moisture. The experimental variable is the planting depth. Observe daily and note which ones germinate.
You can conduct similar experiments with seed germination simply by changing one variable and keeping other variables constant. For example, you can plant all the seeds at the recommended depth but vary the temperature. You could vary just the moisture, showing how in nature some seeds may not germinate if sufficient moisture isn’t present.