Living and Nonliving Things
What Is This?
Everything in the world can be sorted into two broad categories: living things (also called organisms) and nonliving things. While this might seem like an obvious distinction at first, some objects — like a flickering flame or a slowly growing crystal — can be surprisingly tricky to classify. Scientists use a specific set of characteristics to determine whether something is truly alive.
Why Does It Matter?
Understanding what makes something alive is the foundation of all biology — the scientific study of life. This knowledge helps scientists classify newly discovered organisms, understand how living things are related to each other, and even search for signs of life on other planets. It also builds your ability to observe the world carefully and reason scientifically about what you see.
The Characteristics of Living Things
Scientists generally agree that living things share seven key characteristics. To be classified as alive, something must display all (or nearly all) of these traits.
1. Made of Cells
All living things are made of one or more cells — the basic building blocks of life. Some organisms, like bacteria, are made of just a single cell. Others, like humans, are made of trillions of cells working together.
2. Growth and Development
Living things grow and develop over time, increasing in size or changing in complexity as they mature — a seedling growing into a tree, or a kitten growing into a cat.
3. Reproduction
Living things can produce offspring, passing on their traits to a new generation, ensuring the continuation of their species.
4. Response to Stimuli
Living things react to changes in their environment, called stimuli. A plant bending toward sunlight, an animal flinching from a loud noise, or your pupils shrinking in bright light are all examples of responding to stimuli.
5. Use of Energy (Metabolism)
Living things take in and use energy to carry out life processes — eating food, absorbing sunlight, or breaking down nutrients to fuel growth, movement, and repair. This process is called metabolism.
6. Maintaining Internal Balance (Homeostasis)
Living things regulate their internal conditions to stay stable, even when their external environment changes — a process called homeostasis. Humans sweating to cool down on a hot day is a clear example.
7. Adaptation Over Generations
Over many generations, living things can change and adapt to their environment through a gradual process. This explains why species develop traits suited to their specific habitats over long periods of time.
Characteristics of Nonliving Things
Nonliving things do not display all seven characteristics of life. They may display one or two characteristics (which is what makes classification tricky), but never the full combination.
Examples of nonliving things: rocks, water, air, a chair, a car, a cloud, the sun
A rock does not grow, reproduce, or respond to stimuli — it simply exists and may slowly erode due to outside forces like wind or water, which is different from biological growth.
Tricky Cases: Things That Seem Alive but Aren't
Fire
Fire seems to "grow," "consume" fuel, and even appears to "spread" or "reproduce" by creating new flames — but fire is not made of cells, does not reproduce in a biological sense, and does not maintain homeostasis. Fire is a chemical reaction, not a living thing.
Crystals
Crystals can grow larger over time by adding new layers of minerals — but this growth is a chemical and physical process, not biological growth. Crystals are not made of cells and do not respond to stimuli or reproduce.
Robots and Computers
Advanced robots can move, respond to input, and even "learn" in a programmed sense — but they are not made of cells, do not grow biologically, and do not reproduce on their own. They are sophisticated machines, not living things.
Viruses
Viruses are one of the most genuinely difficult cases in biology. They contain genetic material and can reproduce — but only by hijacking the cells of a living host; they cannot reproduce, grow, or carry out metabolism on their own. For this reason, many scientists classify viruses as existing in a category between living and nonliving, rather than firmly in either group.
Was It Ever Alive? Dead Organisms vs. Nonliving Things
It's important to distinguish between something that is currently alive, something that is dead (was once alive but no longer displays the characteristics of life), and something that was never alive at all.
A fallen leaf, a dead insect, or a piece of wood from a tree were all once part of living organisms, but they are no longer alive. A rock, by contrast, was never alive in the first place.
Classifying Living Things: A Quick Test
When trying to determine if something is alive, ask these questions:
- Is it made of cells?
- Does it grow and develop?
- Can it reproduce (create offspring)?
- Does it respond to its environment?
- Does it use energy (metabolism)?
- Does it maintain internal balance (homeostasis)?
If the answer to most or all of these questions is yes, the object is very likely a living thing.
Common Mistakes
Mistake 1: Assuming movement means something is alive
Many nonliving things move — clouds drift, rivers flow, cars drive — but movement alone does not make something alive. Living things must display the full combination of characteristics, not just one.
Mistake 2: Confusing "was once alive" with "is currently alive"
A piece of wood, a leather shoe, or a cotton shirt all came from living organisms originally, but they are not alive themselves — they no longer display the characteristics of life.
Mistake 3: Thinking growth alone proves something is alive
Crystals and even some geological formations can "grow" over time through nonbiological processes. Growth must be combined with the other characteristics of life, like cellular structure and reproduction, to indicate something is truly alive.
Key Takeaways
- Living things share seven key characteristics: made of cells, growth and development, reproduction, response to stimuli, use of energy (metabolism), homeostasis, and adaptation over generations
- Nonliving things may display one or two of these characteristics but never the full combination
- Tricky cases like fire, crystals, and robots seem alive in some ways but fail the full test of life
- Viruses are a genuinely difficult, in-between case debated by scientists
- Something that was once alive (like a fallen leaf) is different from something that was never alive (like a rock)
Practice and Resources
Ready to practise? Try our Grade 3-5 Living and Nonliving Things Worksheet with classification activities, tricky-case reasoning, and short answer questions. Test yourself with the Grade 3-5 Science Practice Test.