All these busy plants around us, producing what we need to live, and they don't look like they're doing anything. How can we tell if they are performing photosynthesis? One way is to see if they are giving off oxygen, the most important byproduct of photosynthesis. Of course, we can't usually see leaves producing oxygen, but watch what happens when you use an underwater plant!
(Adult supervision recommended, especially with step 5.)
Normally we can't see the oxygen produced by photosynthesis, but when it is produced underwater it appears as bubbles in the water. These float up through the funnel and displace the water in the test tube. Fire needs oxygen to burn, so when you insert the splint, the pure oxygen in the test tube causes it to glow brighter or produce a flame.
The rate of photosynthesis varies with several factors, including the intensity of sunlight and the temperature of the plant. (Other factors include the amount of water and the concentration of carbon dioxide (CO2) in the air.) You can design an experiment to test some of these variables: for example, will photosynthesis happen faster or slower if you put the elodea in warm water? Collect data by measuring how much oxygen is produced in a given amount of time when the elodea is submerged in warm water vs. cold water. You can either mark the level of oxygen on the glass with a wax pencil, or you can use a graduated cylinder instead of a test tube and measure more precisely in milliliters. Try to keep the other variables constant—it's best if you can run two jars simultaneously so you know they are getting the same intensity of sunlight. If you only have one jar, how else could you make sure the light intensity is constant? Could you use a light source other than the sun?