Fire is a transition from one state to another.

For example a block of wood turns into ash and gas. In the process fire and heat occur. The from in this case is wood. The to in this case is ash (a different form of Terra) and gas (Aura).

Religious Catholic paintings show a flame above people's heads. The from variable in this case is ashes. The to variable in this case is ashes. The difference between the two sets of ashes is time, specifically a human lifetime.

Travel, such as running, swimming, flying involves transitions from one location to another. The from and the to variables are locations (Terra) that are not the same. If they are the same the variable of time needs to be introduced. Travel requires energy. Running and swimming burn calories. Cars run on gasoline. That is why these activities belong in the category of ignis, or tranistion.