I'm going to give you a list of words and I want you to tell me which of these things are alive, and which are not. Okay, here it is: dog, rock, people. If you picked "dog" and "people" you're right. I bet that question was too easy for you.
Why did you pick "dog" and "people" but not "rock"? What are some things that dogs and people do that make you think they are living? We know that dogs and people move around, eat food, breathe air, and grow. Since rocks don't do any of those things, they must not be alive.
Rock - not so alive
Does something that is alive need to do all of those activities on your list? Plants do not walk, and very little babies and people who need to stay in a wheelchair can't walk, but I'm sure that you would consider them alive. That changes the list we made to explain why dogs and people should be considered living things. Let's make a new list of words that describe something that is alive: eat food, breathe air, and grow.
Food gives plants and animals the energy they need to grow and stay alive. Food also gives animals the energy they need to move around and think. You might feel too tired to play or run when you are very hungry because you do not have enough energy to do those activities. Trees and flowers definitely grow, but do they eat food? Sort of. Plants do not have mouths, so instead of eating like we do, they soak in water and other chemicals that they need from the soil. The water and chemicals that plants soak in help them make their own energy.
Roots are thirsty
How do plants soak in the water and chemicals? The next time you take a shower or wash the dishes, place a dry sponge at the bottom of the tub or sink. At the end of the shower, or when you are done cleaning, look at the sponge. I bet you'll see that it has soaked up a lot of water. The roots of plants do the same thing. Plants can then use the water and chemicals that they soak up to make the energy they need to grow.
Now let's talk about breathing. People and animals use their nose and mouth to breathe in air. How do you think plants breathe in air, since they do not have a nose or mouth? Underneath a leaf from a flower or tree there are teeny, tiny holes. These holes are where air goes in and comes out.
Not everything alive breathes, though. There are living things at the bottom of the ocean that live near underwater volcanoes. The water there is so hot that people would die if they swam up to these living things. These creatures don't stay alive the same way as plants and animals. Instead, they use the energy from the heat to get the energy they need to stay alive. Now it's getting harder to say what it means to be alive. We think that to be living means you need energy, work to stay alive, and that you grow.
We have decided that in order for something to be considered alive, it needs to use energy, work to keep itself alive, and grow. That means we can say that if something does not need energy, does not work to keep itself alive, or does not grow, it is not alive. A rock does not need energy, it does nothing to protect itself or stay as a rock, and it won't grow--no matter how much you try to feed it. We can call things that are not alive non-living things.
Scientists call anything that is living an organism. If anything that is living is called an organism, that means that animals, plants, and people are all organisms. The next time you hear a dog barking, you can say "The organism is barking." If you say that, the people around you will think that you are super smart!
All organisms are made up of small pieces called cells. Have you ever played with a puzzle? You can think of the puzzle as the organism and all the puzzle pieces as cells. Since you are an organism, you are made up of cells. However, just like those holes that are underneath leaves, your cells are super-duper tiny and much too small to be seen without using a microscope.
Does everyone in a school do homework, teach, serve lunch, and drive the bus? No, of course not. Each person has a different job or task. In a school, students learn, teachers teach, the lunch staff serves lunch, and the bus driver drives the bus. When everyone has their own job to do, they can put all of their energy and time into doing that job. You might not do as well on your classwork if you had to spend time serving lunch or driving the bus too.
Your body divides up the work of living just like people divide up the work of running a school. Each task is given to an organ. The heart pumps the blood, the lungs bring air in and out of the body, and our brains think. Since each of these organs has a different job, they need to be made up of different cells. The cells of these organs are slightly different from each other because they need to do different tasks. However, they are still called cells. If you are confused, think about people. Even though people can be tall or short, funny or serious, we still call all of them people.
Ya gotta have heart
What have we learned today? We learned that almost all living things breathe, grow, and need energy. There are a few organisms that do not breathe, but we can talk about them later. We also learned that all living things are called organisms, and all organisms are made up of tiny cells. Finally, we learned that there are many different types of cells, and that they all do a different job for our body.