Something may not work if some of its parts are missing. When parts are put together, they can do things that they couldn't do by themselves. They are not the same size, are missing many details, or are not able to do all of the same things. A model of something is different from the real thing but can be used to learn something about the real thing. One way to describe something is to say how it is like something else.
People can keep track of some things, seeing where they come from and where they go.
Things can change in different ways, such as in size, weight, color, and movement. Some small changes can be detected by taking measurements. Some changes are so slow or so fast that they are hard to see. Things in nature and things people make have very different sizes, weights, ages, and speeds. A primary class examines a river rock mixture containing earth material particles of various sizes.