What happens to a plant growing in air with extra carbon dioxide? To find out, you germinate seeds in two peat cups with potting soil, then cover each with a large soda bottle placed in a well-lit area. One bottle contains a dish of baking soda and vinegar, a mix that produces CO2. The other has no CO2 source at all, making it the control. Both setups get the same light and soil. Each day for five days you record plant growth and temperature inside each bottle, along with any visible water vapor. Comparing the two tells you how extra CO2 affects growth and temperature, because every other condition is identical.

To test the greenhouse effect fairly, you need two setups that start identically. Place two thermometers side by side and wait until both show the same starting temperature. Then cover one with a glass jar, leaving the other open to the air. As the minutes pass, the covered thermometer climbs higher and higher while the uncovered one stays close to where it started. Because only the jar differs between the two, any temperature gap you see must come from the jar trapping heat.

This experiment shows how a control keeps a test fair. You place sprouted seedlings in two soda-bottle chambers set up the same way. One bottle gets baking soda and vinegar to release carbon dioxide, while the other has no added gas and serves as the control. Because only one thing differs, you can trust that any growth difference came from the extra CO2.