When sitting and watching a fire, it is easy to get the idea that it is alive. It moves dynamically, dancing and sending sparks. It is able to spread across a surface, and move about from combustible object to combustible object. It consumes such materials as wood, converting them into ash and other byproducts. It needs oxygen, as though it were breathing. However, fire is most definitely not alive.

In fact, none of these criteria prove that something is alive!

Oftentimes we think that all life requires oxygen. However, there are many bacteria (called anaerobic bacteria) which do not need oxygen at all! They live down in the soil, under oceans and lakes, and in other areas. To many of them, oxygen is a poison! Yet they are definitely alive. So clearly whether or not something needs oxygen does not show if it is alive.

Consciousness as we think of it is probably not a good determinant for life either. It is highly unlikely that bacteria, fungi, plants, and other organisms think, since they lack a brain or nerves. Yet they are definitely alive.
Therefore whether or not something thinks does not show that if is alive. This is the same for movement, because not all living things move-- in fact, many living things move a lot less than fire! And many non-living things move, such as the wind or a car.

In order for something to be considered alive it must have several characteristics and abilities, such as metabolism, growth, reproduction, and response to stimuli or adaptation to the environment. How does fire stack up to these requirements?

Fire certainly seems able to grow. A small campfire can quickly grow to the level of a forest fire, so yes, it seems to meet this criterion.

Can fire reproduce? Again, it seems able to! A single spark can blow to a different location and start a new fire. That might be considered reproduction!

Fire is not alive, although it certainly seems to have a mind of its own! The process of oxidation is when substances react with the oxygen in the air to make oxides (like rust on iron, is iron oxide, and when your mom's silver tarnishes, that is silver oxide). When things oxidize, they release heat. Fire is just very rapid oxidation. Heat rises, and makes currents of air that circulate as the hot air rises - that is why flames appear to dance and move around. Fire needs oxygen to burn, and it needs fuel; for something to "burst into flames" it needs to be at a high enough temperature. Without any one of these ingredients the fire won't burn.
A flame on earth is in a gravitational field - that is why it is thicker at the bottom. But a flame in the space shuttle, in what they call "microgravity"(*) environment is actually round! Pretty amazing - imagine a "flame ball"! (* Actually there is plenty of gravity up there - that is why the shuttle stays in orbit - but it is in a state of "free fall" around the earth so you _feel_ weightless.)

Biologists have fought a bit over the basic definition of life, but all biologists would agree that fire is not alive. Remember, not all living things feed on oxygen (plants feed on carbon dioxide), so that's not a good definition for life. Not all living things move (again, plants do not really move), so that's not a very good definition of life. Not all living things "think", either (plants do not think).What your question really comes down to is how to define life. I've tried to come up with some basic definitions of life.
-- For me, life can be defined by it's most basic building block -- a cell. If something does not contain at least one cell, it is not alive. Fire does not contain cells.
-- Living things contain DNA and/or RNA, proteins which contain the basic information cells use to reproduce themselves. Fire does not contain DNA or RNA.

-- Living things are made of matter, and you can weigh them. It may be hard to measure the weight of one bacteria, but you get the point. You cannot weigh fire, because it is simply energy. It has no mass. Fire is energy given off when matter burns. You can see it, but what you see is merely light, so you cannot hold it.
-- Living things grow, and in growing they make new matter, but it's not just any old matter.
Life is organization. All living things make the same basic things: proteins, fats and carbohydrates, for example. Fire cannot make proteins from the oxygen and hydrogen and carbon it "feeds on", just destroy proteins. This is a major difference.

-- Living things require nutrients and water, and have complex ways of finding and using the things in their external environment that they need. Living things also have ways of sensing and responding to threat or attack.

That's a great question because fire does have some things in common with living things. It needs fuel and oxygen. It can grow. It "reproduces" to make more fires.
But fire is also different from living things. For one thing, it is not made of cells. All living things are made of cells.
Also, when fire "reproduces," no information is passed on. In living things, DNA carries information from one generation to another. All living things are adapted to where and how they live by evolution, which changes DNA over time. Fire is basically the same every time. It may be bigger or smaller, hotter or less hot, or moving differently, but that's all because of the conditions right now, not because of information it inherited in DNA.
Another difference is that all living things come from other living things. One-celled living things divide to make two new one-celled things. Eggs, spores, seeds, babies, and growing new individuals from parts of their "parents" are all ways to reproduce new life from live things. Fire can come from a match, rubbing sticks together, a spark, or other things that are not fire.
So fire is a great example of something that has some characteristics of life. Thinking about ideas like this helps us to really figure out what we mean by life.
If you could make robots that made other robots, would they be alive?

According to Hickman, Roberts, and Larson (1997), any living organism will meet the following seven basic properties of life:
1) Chemical uniqueness. Living systems demonstrate a unique and complex molecular organization.
2) Complexity and hierarchical organization. Living systems demonstrate a unique and complex hierarchical organization.
3) Reproduction. Living systems can reproduce themselves.
4) Possession of a genetic program. A genetic program provides fidelity of inheritance.
5) Metabolism. Living organisms maintain themselves by obtaining nutrients from their environments.
6) Development. All organisms pass through a characteristic life cycle.
7) Environmental reaction. All animals interact with their environment.
Fire is not alive. It does not contain genetically inheritable information, and arguably does not have a complex organization.

While it is true that fire feeds on oxygen and moves it is lacking a very important feature to be considered alive. In order to be considered alive you have to be able to reproduce either sexually or asexually at some point in your life. All living things also contain a genome (DNA), that is transferred to it's progeny (offspring)