Great question. You probably have some ideas of your own already. If you see something moving on its own (not blown by the wind, moved by the water, etc.), you probably have a good idea that it's alive. If you see it growing, that's another good clue.

Animals like us, plants, bacteria, fungi, and the one-celled organisms we call protists are all alive, but we do some things in different ways.

Here are some things that biologists look for in all living things:

We all grow. We all need to take in matter and energy. We may eat to get both matter and energy or use sunlight for energy and bring in matter through leaves and roots. We all need to exchange gases with the environment. We take in oxygen and give off carbon dioxide. Plant cells do that when they can't do photosynthesis. When they can do photosynthesis, they take in carbon dioxide and give off oxygen. All living things reproduce, but it works very differently in some living things. Some can divide in half or make little "mini-me" clones.

We are all made of cells. We all have DNA, which contains all the information it takes to make a living thing.

Viruses have some of the characteristics of life, but not all of them. They are not cells. They can only reproduce by taking over a cell.

Why isn't fire alive? Think about which characteristics it has or doesn't have.

Shyann, great question. Here's a list of 9 questions scientists ask to know if something is alive!

1. Is it made of cells?
2. Can it have babies, and does it grow?
3. Can it evolve or adapt or change over time?
4. Can it heal itself?
5. Is it made of the same basic chemical ingredients, or stuff, as you?
6. Does it need water and food?
7. Will it die?
8. Does it change throughout the year?
9. Can it move?

Useful Source with pretty graphics!
something is alive or dead.

A few of the key attributes of living things are the following:

-they can reproduce
-they use energy
-they are made of cells
-they respond to stimuli
-they can grow
-they can transport nutrients throughout their body
-they excrete waste

I think a good example of applying these points is to look at a virus. At first glance you may think a virus is living, as they can spread across the globe, cause us significant harm, and we go out of our way to “kill viruses”. However, technically speaking, viruses are not alive for the following reasons:
-They cannot replicate on their own- a virus does not contain the proper “tools” to replicate its own genetic material- it infects a host and uses its host’s cells to do this.
-They do not grow- when a host is infected, they produce new viruses in their fully formed state
-They do not use energy

Great question! The scientific definition of "alive" has actually been debated for many years, and in some cases is still controversial. There are some basic qualities of a living thing that we can generally agree on, for instance: having an organized structure, being able to reproduce and grow independently, responding to stimuli, maintaining homeostasis (specific conditions under which the organisms best functions), and having a metabolism. That is why your dog who grows and eats and reproduces is alive, while your pencil, which does none of those things, is not.

Even single celled organisms are structured, grow with time, and do all the things necessary for life. However, this is where we run into some gray area. For instance, are viruses alive? They replicate, have DNA, respond to stimuli, have a metabolism, and maintain homeostasis. The difference is that they cannot accomplish many of these functions (i.e. reproduction and metabolism), without the help of a host cell they infect. That is why the concept of "independent" life is so important, and so hotly debated. Thanks for your curiosity!

All life that we understand has chemical reactions inside of it that produce energy that the life-form uses to grow and reproduce. Also, this reproduction happens in a controlled way, not just random spreading like a fire does.

There are some cases that make us wonder, though.

What a fun question. You need to know what 'life' is. Scientists have argued about that question a lot.

If you think it's hard to know what's alive on earth, imagine being on another planet and trying to figure out if there's life! It's not easy to tell if something is alive.

We used to say viruses aren't alive, because they can only make more of themselves when they're inside a living cell. But they're different from non-living things, because they do have DNA or RNA with instructions for making more of themselves. They're like parasites - they need to be inside something living to be able to reproduce. So they're sort of alive.

You can probably tell if seeds are alive or dead by planting them and seeing how many of them sprout. Sometimes very old seeds can sprout.

Other living things can be dormant - in a deep resting state. I'll just give you the Wikipedia link -
Dormancy.

There is no universally-agreed set of conditions which define being "alive". Many use the following set of seven characteristics, with something needing to have or display all to be alive.

1) Maintain homeostasis - keep internal conditions which are different from the surroundings.

2) Have organization - a non-random structure, sometimes listed as being composed of cells (and usually also required to be hierarchical, meaning building more complex pieces from simpler components).

3) Have metabolism/require energy - transforming energy from one form to another.

4) Adapt - ability to change with the environment.

5) Respond to stimuli - taking action as a result of some happening in the surroundings, such as moving away from something causing pain.

6) Reproduce - ability to make a new generation of the same type of living thing, passing on genetic information.

7) Grow - ability to increase in size.

There are numerous variations on this set, some with only subtle changes to wording, for example having complex chemistry vs. metabolism, or Improvisation vs. responding to stimuli.

Since over 100 definitions of "alive" have been used in relatively recent scientific publications, there are some with greater differences as well. Some include criteria such as movement, having genetic information stored as DNA, or being carbon-based ( here, after the list of typical criteria). Although commonly held, these criteria can lead to humorously incongruous conclusions. For example, as noted in an anecdote related by Dr. Koshland (The Seven Pillars of Life - link above) , such as that a single rabbit must be dead. After all, two rabbits (one male and one female) are required for reproduction, so only one would not meet criterion #6 above, and therefore not be alive. (He suggests an amended #6, "ability to be at least a partner in reproduction".) Or, certain types of crystal particles suspended in a liquid can meet criteria for life, but most would not consider them to be alive. Viruses also present a quandary, with some maintaining that they are not alive because they do not metabolize and cannot reproduce on their own, but others being adamant that viruses are alive because of slightly different criteria for life.

Many other questions about "life" have been asked/answered on ScienceLine and may be of interest. ( Origins of life ; viruses and prions ; fire ; wood ; seeds .)