It is difficult to burn a heap of green leaves as green leaves contain moisture i.e., they have water in them on the surfaces of the leaves and we know that water is non-combustible that is it does not catch fire easily so the leaves don't catch fire easily. But in the case of dry leaves there is very little moisture or no moisture left at all so the dry leaves can burn easily.
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Because green leaves contain more moister than dry leaves they catch fire easily.

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