Minecraft wiki entry for Water, associated text: Calculates a local_soup_heat equal to the sum of the 3x1 soup_heat neighborhood around the current element.
Calculates the new soup_heat as the sum of the local_soup_heat divided by 3.3F plus the pot_heat times 0.8F
Calculates the new pot_heat as the current pot_heat plus the flame_heat times 0.05. pot_heat is clamped to a minimum of 0.
Calculates the new flame_heat as the current flame_heat minus 0.1F.
However, there is a 0.05 in 1 random chance that flame_heat is set to 0.5.