Have you ever wondered how scientists know when the population of a species is increasing or decreasing? Do they go out and count every single one to see how many there are? Or is there an easier way to do it?
Understanding chemical and physical changes is a strange part of science. Once you learn to use chemical equations, it is so simple that no one even thinks about it. Before you learn to use chemical equations, it is so complex that most people don't really understand it. For a better view of that, you might also read Changing How We Look at Changing.
If you look at the label of any food, you will probably find that it lists the Calorie content. Calories tell us how much energy you will get by consuming the food. How do scientists measure that energy?