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[Phys-l] Crackling of a fire



A student has posed this question to me, and I'm not sure I have a good
answer, so I am seeking help...

The Question:
Why, when you put a log on the fire does it 'pop, crack and crackle'?

My proposed answer:
I think the answer lies in the sap content, even i dry wood. As the wood
gets hotter, the sap trapped within the fibers heats and then boils
bursting forth, breaking the fibers giving the 'crack' sound. Sap trapped
at the surface would expand and then burst giving the 'pop' sound.

Refinements, clarifications, and debunking are welcome!
Thanks,
Peter Schoch