It is pretty clear to most people, even very small children, that kitchen pots are not alive. These objects are cold to the touch (unless they've been on the stove); they don't move on their own; they don't reproduce. At the opposite end of the spectrum, it is also clear that lions, dogs, and human friends are alive, especially when you see them run, jump, or respond to you. Those are fairly easy examples, right? But what about a cell phone? Or a stick of firewood? Are these things living or nonliving, and how can you tell?

You know that a cell phone is nonliving, right? But it makes noise and it responds when you press its buttons. That's more lifelike than a pot! And what about firewood? Firewood comes from trees, and trees are living. But if a tree dies and is chopped up for fuel, are the logs from that tree nonliving because they are no longer alive? As you can see, the distinction is not always clear-cut. Some inanimate objects have characteristics of living organisms, while many living organisms, on the face of it, seem utterly lifeless.

So how does one distinguish between living and nonliving things? First of all, the scientific definition of living includes those things that are alive or have ever been alive -- including that stick of firewood. In contrast, nonliving things are not alive, nor have they ever been.

But what does it mean to be alive? According to biologists living organisms are characterized by seven "signs of life": 1) living things have highly organized, complex structures; 2) living things maintain a chemical composition that is quite different from their surroundings; 3) living things have the capacity to take in, transform, and use energy from the environment; 4) living things can respond to stimuli; 5) living things have the capacity to reproduce themselves; 6) living things grow and develop; and 7) living things are well-suited to their environment.

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