The third boon was the still more secret teaching of conquering death. Yama then tempted Naciketas with all kinds of other things, but the youth remained firm, and in the end Yama granted him his desires. One must be willing to give up everything to attain immortality. According to the version of this story told in the Katha-Upanishad, Naciketas received his wisdom from the sacrificial fire directly. Exactly in that way.
Fire is a living reality.
Don't think of fire only as what you would call a chemical process. Nothing in this world is quite what it appears to be, least of all fire. You need to look at the world differently. Your eyes don't see deep enough. To you people, everything is dead matter until you come to plants.