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        What Is Life?

        In this lesson, students observe a number of objects, make a list of life’s characteristics, and develop a working definition of being alive.

        Lesson Summary


        Students observe a number of objects, make a list of life’s characteristics, and develop a working definition of what it means to be alive. Using a confounding example, students test their definition and conclude that one needs multiple characteristics to define and search for life.

        This activity was adapted from:
        Astrobiology Education Poster | NASA Astrobiology Institute
        From Activity 1 in Life on Earth ... and Elsewhere?

        What Is Life? | NASA Astrobiology Institute
        Activity 1 from pages 5–10 in Life on Earth ... and Elsewhere?, grades 3-4.


        • Learn that by understanding the characteristics of living things, scientists can develop appropriate tools to search for life beyond Earth
        • Discover that there is no single indicator of life
        • Understand that to define something as alive, we must look at many characteristics collectively

        Grade Level: 1-6

        Suggested Time

        • One class period (approx. 15 minutes)

        Media Resources


        • What Is Life? Presentation Slides
        • Poster paper and a marker or a drawing board
        • A pair of objects per student (or per every two students); for example:
          • A real piece of fruit and a plastic replica (from a flower, Christmas, hobby, or decorator shop)
          • A live flower and a similar kind of silk, paper, or plastic flower
          • A live leaf and a similar kind of silk, paper, or plastic leaf
          • A live tree leaf and a dead tree leaf (the same kind)
          • Live grass and dead grass (the same kind)

        Before the Lesson

        • Download the What Is Life? presentation slides.
        • If you are distributing pairs of objects, put them both in a bag or container.

        The Lesson

        Part I: Engage

        1. Ask:

        • Who is alive? (All hands go up!)
        • Who can tell if something is alive? (Most likely, every student will raise his or her hand.)
        • How do you know something is alive? (By what it does or how it acts.)
        • What are some things all living things do? (Grow and develop; consume raw materials [eat and drink], use nutrients, require energy; produce waste; reproduce; respond to changes in the environment; consist of one or more cells; and evolve. Remember to include only those characteristics that make sense for the age group you are working with.)

        Part II: Facilitate

        2. Give students an object or pair of objects: Have them look at the object(s) and list observations that indicate whether or not the object is/was alive. (You could also do this as a demo.) Remember that you are working toward a definition or criteria for life. As a group, develop a list of characteristics on a drawing board.

        3. Show students the What Is Life? presentation slides of living and nonliving things and ask them to call out, “It’s alive!” when they see something that is living.

        4. Ask:

        • Is a piece of fruit or blade of grass alive? Can you say something is alive if you only have a piece of it and the piece really couldn’t survive on its own? (A piece of something that is/was alive is evidence of life, and would qualify as real evidence for scientists looking for life.)
        • What about fire?—a confounding example (This is optional, depending on your time and the audience’s age. Point out that a wildfire uses raw materials, moves, grows, produces waste, responds to its environment by changing direction with the wind or going out in the rain, and reproduces by its sparks starting new fires.)
        • What are some things that are true for living creatures but not for fire? (Organisms are self-contained chemical systems with consistent shapes and predictable behaviors. Importantly, they can produce offspring that exhibit genetic variation.)

        Check for Understanding

        Tell students to hold up an item that they believe is alive.

        Ask: How hard is it to tell which item was real and which was the copy? How did you know?

        Conclude by saying that one needs multiple characteristics to detect life. Help the students see that it isn’t just one characteristic that defines something as alive. Point out that it is difficult to define something as alive by just one or two characteristics. For example, movement alone doesn’t mean something is alive—a ball can roll down a hill. Mention the importance of long-term functions, and that many characteristics occur for a short time period. But to persist, life evolves and adapts to changing conditions over long periods of time.

        Keep Exploring

        Observe another confounding example, sewer lice (Optional)

        This classic demonstration is an opportunity to observe characteristics often associated with living things exhibited in nonliving materials. View the Sewer Lice (PDF).

        Watch Life’s Basic Ingredients (Optional)

        The accompanying Life's Basic Ingredients video (three minutes long) highlights the three main ingredients that life needs and ends with a mention of comets as a possible key source of organic materials.


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