What Is Life
Look at a rock and a ladybug sitting side by side in a garden. One of them is alive and one is not — but how do you actually know? Scientists who search for life on other planets, called astrobiologists, have to answer that exact question every single day, because someday a robot might send back a strange photo from Mars and someone will have to decide: is this alive?
What You'll Learn
- Learn 4 things that living things all do - Compare a living thing and a non-living thing - Meet astrobiologists, scientists who search for life in space - Practice sorting things into living and non-living
Four Things Every Living Thing Does
Scientists agree that living things do these things: they grow, they use energy (like eating food or sunlight), they respond to their surroundings (like a plant turning toward light), and they can make more of themselves (babies, seeds, or eggs). A ladybug does all four. A rock does none of them — it just sits there, even for a million years.
Meet the Astrobiologists
Astrobiologists are scientists who study whether life could exist on other planets and moons, like Mars or Europa (one of Jupiter's icy moons). NASA's Perseverance rover has been driving around Mars since 2021, drilling into rocks and looking for tiny signs that something might have grown, used energy, or left behind a trace long ago.
Space life might not look like Earth life at all — it could be too small to see, or built from different chemicals. That's why astrobiologists look for the four life clues (grow, use energy, respond, reproduce) instead of just looking for something that looks like an animal.
Living or Not? Some Tricky Ones
Fire grows and uses energy, but it can't make more fire on its own and doesn't respond the way living things do, so it's not alive. A seed looks like a tiny rock and doesn't move, but it's alive — it's just waiting for water and warmth to grow. Being tricky to sort is exactly why scientists use all four clues together, not just one.
Why is fire NOT considered a living thing, even though it grows and uses energy?
What do astrobiologists mainly study?
Living or Not Living Sort
Walk around your home or yard and find 6 items. For each one, check off whether it grows, uses energy, responds to surroundings, and can make more of itself. Draw two columns labeled 'Alive' and 'Not Alive' and sort your 6 items based on how many checks they got.
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