Difference in pressure can be used to move materials/liquids. There are numerous application areas ranging from medical to mechanical, which owe their success to this principle. Knowing what can be done to control the pressure difference is key to understanding and designing an efficient system. It was really interesting when children go beyond just observing an experiment and start thinking about what affects the outcome. 

Such open-ended questions to an experiment is what truly makes it successful. What is the dominating factor that creates the pressure difference inside the glass – temperature inside the glass or amount of oxygen that is used up by the burning candles? Or do both play an equally important role?