Because they gather into one point, or focus, several rays of caloric as they are travelling from the sun, and the accumulation of caloric developes that intensity of heat which constitutes fire.
In optics, it is the point or centre at which, or around which, divergent rays are brought into the closest possible union.
"Yet man is born to trouble, as the sparks fly upward.—I would seek unto God, and unto God would I commit my cause."—Job v.
It is a violent chemical action attending the combustion of the ingredients of fuel with the oxygen of the air.