What if one group had Harvest Day apple juice and another had Minute Made and one turned cloudy and the other didn't. Usually, an experimental question will generate many more questions. Have students make pinhole cameras with coffee cans. Identify each camera by number and measure the diameter of the pinhole. Make a hypothesis before doing the experiment.
Student groups test each camera by looking at a single light bulb in a darkened room.
After recording their results, hold a class discussion to formulate a conclusion. Mara's experiment, The Mystery of Reflection is also fun. You could try this one but attempt to answer a question instead. With this experiment, you can demonstrate the importance of a large sample size. It encourages them to think and learn how to properly solve problems.