Fire is actually a gas, just like the air we breathe. Even though it doesn’t like it is floating away it is. Fire is hot gas and rises upwards and you can’t see it at a certain point because it stops releasing light (the picture above demonstrates the top of the flame vanishing from sight.)

Fire is a chemical reaction between a fuel which contains carbon and oxygen. The chemical reaction only happens when the fuel is heated to a certain point (about 150 degrees Celsius or 300 degrees Fahrenheit). The chemical reaction releases gases, which vary depending on the fuel, which are usually a mixture of hydrogen, carbon and oxygen. Fire releases light which is why you can see it, the invisible parts of flames are the hottest (not the part at the top though that’s different) followed by blue and the red/yellow mixes are the coolest. Fire also releases heat, which results in more chemical reactions and keeps the flame burning.

The usual explosions that are big and flamey (like those from bombs in movies) are just fuels burning really quickly.

In space fire doesn’t rise up it, goes in all directions.

You can move your finger through a candle because it takes a long time for gases to heat up other matter (matter in this case being your finger) but the fuel (the thing on fire) will burn you because it is solid (well usually solid).

A bit of personal experience has taught me eyebrows can be permanently made uneven from being singed by fire (one of mine is now visibly thinner than the other and looks odd).