Everyone has seen fire turn wood to ash. Nothing grows from ash — no seed, no sprout, no new tree. Krishna uses this everyday image to make the deepest point of the chapter: just as a blazing fire reduces firewood to ash, the fire of knowledge reduces all karma to ash. Once burned, karma cannot germinate again.
The simplicity of this metaphor is its power. A kitchen hearth, a village bonfire — anyone can picture it. And through that picture, the truth lands: knowledge does not merely manage karma or postpone its effects. It destroys karma completely, leaving nothing behind that could bind the knower ever again.