Plants take in carbon dioxide to produce food. The hypothesis is that if a plant does not get carbon dioxide, it will become unhealthy and possibly even die. Wrap a plant in clear plastic and seal it with tape so that it is air-tight. Run a small plastic aquarium tube from outside the wrap through a hole poked in the wrap, with the other end of the tube set into the plant's soil. This will allow you to water the plant without removing the wrap. Clamp the end of the water tube when not in use. Simultaneously, grow a similar plant next to the wrapped one to act as a "control." Monitor both plants daily. The result will show that the carbon dioxide-deprived plant will eventually appear less healthy.
We turn on a lamp to give us light. However, incandescent bulbs also give off a lot of heat, which is a byproduct and a waste of energy. Hypothesize that the heat given off from an incandescent bulb is much greater than the heat given off by an energy-efficient CFL (compact fluorescent) bulb. Place a thermometer on the glass bulb of a 40-watt incandescent light bulb and measure the surface temperature. Place a thermometer on a CFL bulb that is rated with the equivalent light output as a 40-watt incandescent bulb. The result will be that less heat is given off from the CFL bulb. Take your project one step further by checking the temperature of an equivalent LED (light emitting diode) light, which is reported to be even more energy-efficient than a CFL bulb.
Step outside on a clear night and you'll see hundreds of points of light in the sky. If you use a telescope, however, those hundreds of lights suddenly become thousands of lights. Hypothesize that what looks like a single light point at a distance can actually be two or more individual light sources. Place two flashlights side by side and set them on a porch railing or step on a dark night. Walk several blocks away and look back at the lights. The result will be that you see what appears to be one single light source. See how close you need to get before your eyes can detect that you are actually looking at two separate light sources.
Living organisms require rest. Hypothesize that a plant that gets constant light will not be as healthy as a plant that receives the usual cycle of daylight and darkness during a 24-hour period. Germinate identical seeds in containers. Place one under an artificial grow light that is left on all of the time. Place the other plant where it receives normal light during the day and darkness at night. Observe the plants for several weeks. The result will be that the plant that doesn't get any rest ("sleep") will not appear as healthy as the other plant.