According to the Muggle definition of combustion, it was an exothermic reaction in which a combustible substance reacted with an oxidizing agent, usually accompanied by heat, flames, light, or smoke. Combustion required fuel, a certain amount of oxygen, and ignition energy—for example, adding logs to a fireplace made the fire burn more fiercely.
Floo Powder—whose main ingredient was an extract from the flowers of a magical plant called Floo, rather than merely its pollen—not only turned fireplace flames green when it came into contact with them, but also rendered the fire harmless and no longer scorching.
Some spells also produced flames, light, and similar effects, but they were not all caused by combustion, like the red light of Expelliarmus.
But the magic the children would be using today was a genuine combustion spell: Incendio Duo, which Pomona only taught in seventh-year Herbology. It was highly effective against Acromantula webs and Meat-eating Slugs. Though potions were sold in Diagon Alley, she needed to find something for the children to do, and with all that excess energy, why not have them clean up the spiderwebs in the Forbidden Forest?
After the Basilisk died, the spiders' natural enemy was gone, and they had clearly begun breeding faster. When the wind dragon flew over the Forbidden Forest the previous night, she had noticed misty webs covering the treetops throughout the center of the forest.
The children were excited. Playing with fire was ultimately much more fun than building blocks according to incomprehensible blueprints, though Pomona was somewhat worried that they would set the forest ablaze.
Albus took an almost indulgently caring attitude toward Hagrid. The 1965 Ban on Experimental Breeding made creating new magical creatures illegal, yet Hagrid had successfully bred Fire Crabs and Manticores, creating Blast-Ended Skrewts.
Never mind how he had managed