Teaching: What makes something alive
Read this together, 257 words
Looking for Life on Mars
On July 20, 1976, a NASA spacecraft called Viking 1 set down on a wide plain on Mars named Chryse Planitia. A second lander, Viking 2, touched down farther north about seven weeks later, on September 3. Each one carried a small laboratory built to answer a hard question: is anything alive in the Martian soil?
The scientists could not count on seeing a creature. They had to decide ahead of time what living things do that nonliving things don't, and then test for it. One trait they chose was metabolism: living things take in food and give off gas. In a test called the Labeled Release experiment, the lander added a few drops of nutrient broth to a scoop of soil. Some of the carbon atoms in the broth were radioactive, so if anything in the soil ate the broth and gave off gas, the lander could detect that carbon in the gas.
The first results came back at the end of July, and they looked like life. Gas came off the soil. But another instrument found no carbon-based molecules in the soil at all, which is strange for a place where something lives.
Many scientists came to think the gas came from unusual chemistry in the Martian soil, not from living things. Gilbert Levin, the scientist who led the experiment, argued for decades afterward that it had found life. The question has never been fully settled, and it taught a lasting lesson: one sign of life, by itself, is not enough to call something alive.
Tell Ethan, 1 of 7
Biology is the study of living things, and it starts with a question that sounds easy: what counts as alive?
A dog is alive and a rock is not, but the hard cases are where the definition gets tested. Biologists answer with a list of traits, and a living thing has all of them together.
Tell Ethan, 2 of 7
Living things are made of one or more cells.
A bacterium is a single cell; a person is tens of trillions of them.
Tell Ethan, 3 of 7
Living things use energy.
They take in food or make it, the way a plant makes sugar from sunlight, and use that energy to run the chemical reactions inside them. All of those reactions together are called metabolism.
Tell Ethan, 4 of 7
Living things grow and develop, and they respond to their surroundings.
A seedling bends toward a sunny window. The dark centers of Ethan's eyes shrink in bright light.
Tell Ethan, 5 of 7
Living things keep their insides steady while the outside changes.
Biologists call that homeostasis. A body sweats when it is hot and shivers when it is cold, and both keep its temperature close to the same number.
Tell Ethan, 6 of 7
Living things reproduce, and they pass their instructions, written in DNA, on to their offspring.
Tell Ethan, 7 of 7
The list matters because some nonliving things show one or two traits.
A fire grows and uses fuel. A salt crystal grows in salty water. Neither is made of cells and neither carries DNA, so neither is alive. One trait by itself proves nothing, and the read-aloud, Looking for Life on Mars, shows scientists learning that the hard way in 1976.
Do it together, 1 of 4
Ask Ethan to sort five things into alive and not alive, with a reason for each: a houseplant, a candle flame, a seed in a packet, a cloud, and a cut flower in a vase. Write the answers down and keep them; Ethan will come back to them at the end.
Looking for Life on Mars
On July 20, 1976, a NASA spacecraft called Viking 1 set down on a wide plain on Mars named Chryse Planitia. A second lander, Viking 2, touched down farther north about seven weeks later, on September 3. Each one carried a small laboratory built to answer a hard question: is anything alive in the Martian soil?
The scientists could not count on seeing a creature. They had to decide ahead of time what living things do that nonliving things don't, and then test for it. One trait they chose was metabolism: living things take in food and give off gas. In a test called the Labeled Release experiment, the lander added a few drops of nutrient broth to a scoop of soil. Some of the carbon atoms in the broth were radioactive, so if anything in the soil ate the broth and gave off gas, the lander could detect that carbon in the gas.
The first results came back at the end of July, and they looked like life. Gas came off the soil. But another instrument found no carbon-based molecules in the soil at all, which is strange for a place where something lives.
Many scientists came to think the gas came from unusual chemistry in the Martian soil, not from living things. Gilbert Levin, the scientist who led the experiment, argued for decades afterward that it had found life. The question has never been fully settled, and it taught a lasting lesson: one sign of life, by itself, is not enough to call something alive.
Do it together, 2 of 4
Go back to Looking for Life on Mars, to the part about the Labeled Release experiment.
Ask which trait of life the scientists were testing for, and why one result was not enough to settle the question.
Looking for Life on Mars
On July 20, 1976, a NASA spacecraft called Viking 1 set down on a wide plain on Mars named Chryse Planitia. A second lander, Viking 2, touched down farther north about seven weeks later, on September 3. Each one carried a small laboratory built to answer a hard question: is anything alive in the Martian soil?
The scientists could not count on seeing a creature. They had to decide ahead of time what living things do that nonliving things don't, and then test for it. One trait they chose was metabolism: living things take in food and give off gas. In a test called the Labeled Release experiment, the lander added a few drops of nutrient broth to a scoop of soil. Some of the carbon atoms in the broth were radioactive, so if anything in the soil ate the broth and gave off gas, the lander could detect that carbon in the gas.
The first results came back at the end of July, and they looked like life. Gas came off the soil. But another instrument found no carbon-based molecules in the soil at all, which is strange for a place where something lives.
Many scientists came to think the gas came from unusual chemistry in the Martian soil, not from living things. Gilbert Levin, the scientist who led the experiment, argued for decades afterward that it had found life. The question has never been fully settled, and it taught a lasting lesson: one sign of life, by itself, is not enough to call something alive.
Do it together, 3 of 4
Go through the traits of life together, one at a time.
For each one, ask Ethan for an example from the kitchen, the yard, or Ethan's own body, the way a seedling bends toward a window or a body sweats in the heat.
Looking for Life on Mars
On July 20, 1976, a NASA spacecraft called Viking 1 set down on a wide plain on Mars named Chryse Planitia. A second lander, Viking 2, touched down farther north about seven weeks later, on September 3. Each one carried a small laboratory built to answer a hard question: is anything alive in the Martian soil?
The scientists could not count on seeing a creature. They had to decide ahead of time what living things do that nonliving things don't, and then test for it. One trait they chose was metabolism: living things take in food and give off gas. In a test called the Labeled Release experiment, the lander added a few drops of nutrient broth to a scoop of soil. Some of the carbon atoms in the broth were radioactive, so if anything in the soil ate the broth and gave off gas, the lander could detect that carbon in the gas.
The first results came back at the end of July, and they looked like life. Gas came off the soil. But another instrument found no carbon-based molecules in the soil at all, which is strange for a place where something lives.
Many scientists came to think the gas came from unusual chemistry in the Martian soil, not from living things. Gilbert Levin, the scientist who led the experiment, argued for decades afterward that it had found life. The question has never been fully settled, and it taught a lasting lesson: one sign of life, by itself, is not enough to call something alive.
Do it together, 4 of 4
Return to the five things from the start.
Ask Ethan to check each one against the whole list and change any answer that no longer holds. The seed and the cut flower are the hard ones, so let Ethan argue them out before you move on.
Looking for Life on Mars
On July 20, 1976, a NASA spacecraft called Viking 1 set down on a wide plain on Mars named Chryse Planitia. A second lander, Viking 2, touched down farther north about seven weeks later, on September 3. Each one carried a small laboratory built to answer a hard question: is anything alive in the Martian soil?
The scientists could not count on seeing a creature. They had to decide ahead of time what living things do that nonliving things don't, and then test for it. One trait they chose was metabolism: living things take in food and give off gas. In a test called the Labeled Release experiment, the lander added a few drops of nutrient broth to a scoop of soil. Some of the carbon atoms in the broth were radioactive, so if anything in the soil ate the broth and gave off gas, the lander could detect that carbon in the gas.
The first results came back at the end of July, and they looked like life. Gas came off the soil. But another instrument found no carbon-based molecules in the soil at all, which is strange for a place where something lives.
Many scientists came to think the gas came from unusual chemistry in the Martian soil, not from living things. Gilbert Levin, the scientist who led the experiment, argued for decades afterward that it had found life. The question has never been fully settled, and it taught a lasting lesson: one sign of life, by itself, is not enough to call something alive.
Make something together
Is the yeast alive?
Set out three clear cups or jars and label them A, B, and C. Ethan measures 1/2 cup of warm tap water, about as warm as a bath, into each. Into cup A, Ethan stirs 1 teaspoon of dry yeast and 1 teaspoon of sugar. Into cup B, 1 teaspoon of yeast only. Into cup C, 1 teaspoon of sugar only. Stand a ruler beside each cup. Every 5 minutes for 20 minutes, Ethan measures the height of the foam in millimeters and records it in a table. While waiting, Ethan smells each cup and looks for bubbles rising. Last, Ethan opens the lab notebook and writes the first entry: the date, the question (is dry yeast alive?), what was done, the foam heights, which traits of life the results show, and which traits this test could not show.
- three clear cups or jars
- a marker
- dry yeast
- sugar
- measuring cups and spoons
- warm tap water
- a ruler
- a timer
- a bound notebook to keep as the lab notebook
- a pen
Tip If the list feels like a lot to hold, have Ethan make a short memory line from the first letter of each trait and say it twice. If Ethan moves quickly, ask whether a virus would pass the test, and keep the answer in the lab notebook to check later in the year.
On paper
What Makes Something Alive
8 problems. In your own lessons you print it from here, with an answer key for you.
That’s the lesson
Then you mark it done
In your own lessons, it goes in your child’s record, and a note or a grade helps the next lessons fit. This one isn’t yours, so nothing is saved.
Ethan isn’t real. Arbor writes lessons like this for your kid: their name, what they love, and where they actually are in each subject.