What makes combustibility occur?

Answer 1

there's a couple ways, but mainly exposure to heat with isolated gasses. In simple terms: gas that's stuck gets hot. some things like lighters create a spark(a spark is created by friction, the heating up of things rubbing against eachother, like matches on strikers, usually using flint or some other highly flammable object) that comes in contact with flammable gasses and they catch fire. Other things may be heated to the point that they spontaniously combust.