It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Explanation

Green leaves contain lot of water. So when we try to, burn green leaves, the water contained in the leaves cools them and thus the temperature is brought down below the ignition temperature. This prevents the leaves from burning.
In case of dry leaves there being no water in them to cool, the temperature is raised instantly above the ignition temperature and the leaves start burning.