Animal and Machine
The body of an animal could be compared to some kind of machines, such as the engine of a train. As a matter of fact, the body of an animal indeed is a machine - the machine that is made up of many parts. Each part performs a particular function and has a special structure, which makes it suitable for its function; all the parts are dependent upon each other, and they collaborate to each other to accomplish the overall goal of the machine. The engine must be filled with fuel, such as coal, wood, or other flammable materials. The consumption of the fuel provides the power or energy to the machine. The body of an animal also needs fuel, called food, to provide energy to it. In order to burn the fuel in the engine of a train and oxidize the food in an animal, oxygen must be supplied. The engine of a train is made up of specially designed parts; a furnace that takes and burns the fuel, a pipe that transports steam, wheels that make the engine move, and chimneys that release exhaust gas. Similarly, the body of an animal also consists of different parts: a digestive system that ingests and absorbs food, an excretory system that excretes waste, arteries and veins that transport oxygen and store nutrients, and legs and wings that help animals move.
