Biology studies living things. But how do you know if something is actually living? It's pretty easy to tell that people are living. What about dogs? Or plants? Or bacteria? Is there a sure way to tell? This is one of the problems that biologists study. Biology defines a living creature, technically called a living "organism", as one that has a metabolism and reproduces.

Metabolism is the total of all the chemical reactions taking place in an organism. Some reactions use up energy, and others release energy. For example, when you eat a hamburger, it becomes part of a chemical reaction in your body that releases energy. That's the energy that your body uses to function. Because of metabolism, living organisms grow. This is a very important characteristic of all living organisms.

All living organisms also reproduce. This means that they make a copy of themselves but with variation; that is, their offspring aren't clones but are new individuals. Different organisms have different ways of reproducing. Some single-celled organisms divide into two daughter cells. Some organisms break off small portions of their bodies to form new individuals. Most large organisms reproduce by means of special cells produced specifically for this purpose. All the information necessary to form a new individual is carried by these cells. Because living organisms reproduce with change, their offspring can adapt to their environment. Adaptation is another important characteristic of living organisms.

All living organisms are made out of cells. Cells have even been called the building blocks of life. You can imagine what a cell is like by comparing a living organism to a building. Its cells are like the rooms in the building. Rooms have walls, floors, ceilings, doors, and windows. Cells also have walls, called "membranes", with various kinds of pores, which are like the doors or windows. Both rooms and cells come in various shapes and sizes. They can both have different things inside them, and can be used for their own particular purpose. A building can be one room, or many rooms. An organism can be just one cell, like an amoeba, or many cells, like all larger organisms including humans.

Biology is divided into many smaller fields. Biologists study study living organisms from many different angles, including behaviour, evolution, ecology, cell and molecular biology, and genetics. Biologists study all different kinds of species. Some study insects, while others study humans. Some study plants, while still others study microorganisms. Biology can also be combined with other fields. For example, biochemistry, biophysics, and biopsychology are all important fields related to biology.

Biologists can be found working all over the world, or even in outer space studying the effects of space travel on living organisms. Some biologists work in the field, for example studying the rainforest or birds in their natural habitat. Other biologists may work in a lab studying microorganisms under powerful microscopes. Wherever they are working, biologists can teach us about the amazing diversity of living organisms found on our planet.