Plants are one test subject that are easy to come by and easy to replace, making them an ideal topic for control group science experiments. Elementary students can learn a lot by treating the plant in different ways. One science experiment requiring a control group is to test the effects of milk on plant growth. By giving one group of plants milk and the control group a normal amount of water, students can determine which substance is better for plant growth.
Students themselves can provide a control group as well as a study group in a variety of safe experiments. Not only will these projects get the entire class involved, but it will also demonstrate how science effects them everyday. One science project is to test the effects of cold water on your voice. Using a keyboard to help the singers, determine what the highest note each student can hit with their voice. Then break students into three groups. Ask one group to drink cold water, and again see how high of a note they can sing. Do the same with a group who drink lukewarm water and the control group who don't drink any water at all.
Bacteria and fungi are all around the world--and can also be the subject of many interesting science projects involving control groups. One test elementary school students can do in this area measures how temperature effects mold growth on bread. Students should seal a few slices of bread in containers, such as zipping plastic bags. Place one slice of bread in the center of the room or on the teacher's desk. This will be the control slice of bread. Place other slices in various temperatures, such as in the freezer or in direct sunlight. Students can then observe which slice grows mold the fastest and how temperature may have played a part.
Working with living creatures can offer insight into the natural world of science, and can often be accomplished using insects or animals found in your own backyard. Just always remember to be kind and gentle with living creatures. In one control group experiment, students can determine if treats help cats better learn commands. Take two felines that don't know how to sit on command, for example. Try to teach both cat the "sit" command, while rewarding one's good behavior with a treat. Record and discuss the findings.