Context: Five characteristics are used to define life. All living things share these characteristics. All living things: 1. 2. 3. 4. 5. are made of one or more cells. need energy to stay alive. respond to stimuli in their environment. grow and reproduce. maintain a stable internal environment. Cells are the basic building blocks of life. They are like tiny factories where virtually all life processes take place. Some living things, like the bacteria in Figure 2.1, consist of just one cell. They are called single-celled organisms. You can see other single-celled organisms in Figure 2.2. Some living things are composed of a few to many trillions of cells. They are called multicellular organisms. Your body is composed of trillions of cells. Regardless of the type of organism, all living cells share certain basic structures. For example, all cells are enclosed by a membrane. The cell membrane separates the cell from its environment. It also controls what enters or leaves the cell. Everything you do takes energy. Energy is the ability to change or move matter. Whether its reading these words or running a sprint, it requires energy. In fact, it takes energy just to stay alive. Where do you get energy? You probably know the answer. You get energy from food. Figure {{ref|MS-LS-SE-02-03-Food|below}] shows some healthy foods that can provide you with energy. Just like you, other living things need a source of energy. But they may use a different source. Organisms may be grouped on the basis of the source of energy they use. In which group do you belong? Producers such as the tree in Figure 2.1 use sunlight for energy to produce their own food. The process is called photosynthesis, and the food is sugar. Plants and other organisms use this food for energy. Consumers such as the raccoon in Figure 2.1 eat plantsor other consumers that eat plantsas a source of energy. Some consumers such as the mushroom in Figure 2.1 get their energy from dead organic matter. For example, they might consume dead leaves on a forest floor. When a living thing responds to its environment, it is responding to a stimulus. A stimulus (stimuli, plural) is something in the environment that causes a reaction in an organism. The reaction a stimulus produces is called a response. Imagine how you would respond to the following stimuli: Youre about to cross a street when the walk light turns red. You hear a smoke alarm go off in the kitchen. You step on an upturned tack with a bare foot. You smell the aroma of your favorite food. You taste something really sour. It doesnt take much imagination to realize that responding appropriately to such stimuli might help keep you safe. It might even help you survive. Like you, all other living things sense and respond to stimuli in their environment. In general, their responses help them survive or reproduce. Watch this amazing time-lapse video to see how a plant responds to the stimuli of light and gravity as it grows. Why do you think it is important for a plant to respond appropriately to these stimuli for proper growth? MEDIA Click image to the left or use the URL below. URL: Like plants, all living things have the capacity for growth. The ducklings in Figure 2.4 have a lot of growing to do to catch up in size to their mother. Multicellular organisms like ducks grow by increasing the size and number of their cells. Single-celled organisms just grow in size. As the ducklings grow, they will develop and mature into adults. By adulthood, they will be able to reproduce. Reproduction is the production of offspring. The ability to reproduce is another characteristic of living things. Many organisms reproduce sexually. In sexual reproduction, parents of different sexes mate to produce offspring. The offspring have some combination of the traits of the two parents. Ducks are examples of sexually reproducing organisms. Other organisms reproduce asexually. In asexual reproduction, a single parent can produce offspring alone. For example, a bacterial cell reproduces by dividing into two daughter cells. 
 Question: How many cells make up your body?
Output: trillions