In liquids, particles are closely packed, are not arranged regularly, can slide past one another, and attract and are loosely connected to one another. In gases, particles are far apart, are randomly arranged, spread evenly through the spaces they occupy, move in all directions, and are free of one another, except during collisions. Plants make their own food, whereas animals obtain food by eating other organisms. Decomposers transform dead organisms into simpler substances, which other organisms can reuse. Matter and energy are transferred from one organism to another repeatedly and between organisms and their physical environment.
Several processes contribute to building up and wearing down the earth's surface.
Some of the processes are abrupt, such as earthquakes and volcanoes, while some are slow, such as continental drift and erosion. Matching coastlines and similarities in rocks and fossils suggest that today's continents are separated parts of what was long ago a single vast continent. The solid crust of the earth consists of separate plates that move very slowly, pressing against one another in some places, pulling apart in other places. Major geological events, such as earthquakes, volcanic eruptions, and mountain building, result from these plate motions. Each textbook series was rated according to both its degree of alignment with the selected learning goals and the quality of instructional support in its student and teacher materials.