The Pecular Behavior of Fire

[ApeNotKillApe]

Cold gas is more dense, gravity pulls it down, oxygen feeds the flames and becomes heated, pushing it upward. What you're seeing is the distribution of heat as air is heated, pressured and forced upward through a bottleneck of pressure, sort of like squeezing on a thing of toothpaste.

I see. So fire dances and crackles because the rising heat is fighting against the falling cold air.

The details are naturally more complex, but that's my understanding as a layman.
Crackling is the result of a build-up of pressure in pockets of moisture in the burning material (notably wood) as it vaporizes into steam, expands and bursts out.