The Eternal Candle-Hard Riddles

It wasn’t fire—phosphorescent fungus glowing on the wick

Explanation:

The “flame” wasn’t fire at all but a living glow—a bioluminescent (sometimes called phosphorescent in folklore) fungus growing along the wick. Certain fungi emit a steady greenish light (the old name is foxfire), produced chemically inside the organism rather than by combustion. To an untrained eye in a dim mausoleum the patch of living light would read exactly like a small, constant candle flame—yet it would give off little or no heat, never flicker like a true flame, and could persist for a very long time as long as the wood remained damp and the fungus had nutrients. That slow, unchanging glow is what people took for witchcraft.