Hermione pondered on the question. "What has a life span of a few hours and then die? What expire by serving us? I’m quick when I’m thin and slow when I’m fat. The wind is my enemy. What consider the wind its enemy? What can be stopped by the wind?" There was a moment of silence. "Something tiny could be pushed by the wind, or extinguished like a flame. A flame! That’s it! Okay, what has a small flame that serve someone by expiring? By expiring, that mean it is burning off. Wood, maybe? No, wind doesn’t extinguish a fire – not unless it was very small like it was just lit by a match. A match! It’s close, but it doesn’t last for hours! What does? Wait, I’m stupid, it’s so obvious! It’s a candle!"